Is there a difference between these two words? To me, it seems that undistinguishable is more where you can't tell what it is, and indistinguishable seems to be where they're the same. It seems a lot of places list them as synonyms though.
I'm a native English speaker, and I've never heard of "undistinguishable". I searched for undistinguishable and Google replied with:
Did you mean: indistinguishable
identical: exactly alike; incapable of being perceived as different; "rows of identical houses"; "cars identical except for their license plates"; "they wore indistinguishable hats"
not capable of being distinguished or differentiated; "the two specimens are actually different from each other but the differences are almost indistinguishable"; "the twins were indistinguishable"; "a colorless person quite indistinguishable from the colorless mass of humanity"
I've never seen "undistinguishable" before. My spell-check flags it as an error and suggests "indistinguishable". I suspect it's a typo or a case of misspelling a word in a logical way. I can't imagine that its meaning would be different from "indistinguishable". The Corpus of Contemporary American English (COCA) lists only 8 hits for "undistinguishable" and 1000+ hits for "indistinguishable". I'd stick to the latter.
"Undistinguishable" may perhaps be used only regionally now, I have heard it a lot in my life, but I am from western North Carolina, where Standard English is rarely spoken. It may not be part of contemporary Standard English. Etymonline has an entry for it, listing it from the 1580s meaning 'not distinguishable'.
Look for Califjim's answer. You will be impressed! I pasted it below for your convinience.
There is no rule. Words with these prefixes have come about through accidents of history. The most usual is "un-", but always consult a dictionary. The following does not really answer your question, but you may find it somewhat useful anyway, especially if you're willing to work to dig some of this out of a dictionary.
Dissertation on "Negative Prefixes" in English.
"a-" is a Greek prefix meaning "not" or "without". It is found almost exclusively with words formed from Greek roots. You can usually spot these by the spellings: "ph", "th", "y", "rh", "chr", "pn", "mn", final "sis" or "ic".
theist / atheist chromatic / achomatic rhythmic / arhythmic symmetry / asymmetry
This prefix is found mostly in scientific terminology, especially in the medical sciences. "agranulocytosis", "apnea", "amenorrhea", "anemia", "apraxia", "amitosis".
However, these are not cases where the prefix was applied to an already existing word. Most people know these words as a single unit. They are unaware that the initial "a" has a separate meaning of its own. These should be learned separately, as there are very few pairs like those cited above.
This prefix is also confusable with the native English prefix "a-", as in "ago", "asleep", "aside", which does not have anything to do with negation.
non- has almost exactly the same meaning as "un-", but is less frequent, and here again the best approach is to learn these separately. It occurs more freely with nouns than many of the other prefixes do. Here are a few common ones:
partisan / nonpartisan sectarian / nonsectarian violence / nonviolence standard / nonstandard compliance / noncompliance proliferation / nonproliferation sense / nonsense
The last listed is the only one where the stress shifts to the prefix.
"in-" is a prefix from Latin, so it is usually seen when the root is from Latin. While native English roots tend to be monosyllabic, Latin roots tend to be polysyllabic. "in-"changes to "im-" before "m", "p", and "b". It changes to "il-" before "l" and to "ir-" before "r". This pattern is quite common with adjectives (derived from Latin).
articulate / inarticulate polite / impolite possible / impossible modest / immodest legal / illegal reverent / irreverent regular / irregular sanity / insanity
Latin also uses the prefix "in-" in other ways, not necessarily for negation, so caution is advised! For example, "improve" is not the negation of "prove"! Probably the most maddening of these is the word "inflammable", which means the same thing as "flammable", not the opposite! You will sometimes see the word "nonflammable", which is more clearly the opposite of "flammable".
"dis-" is also a Latinate prefix, but it often means more than the simple negation of "un-". With verbs it may imply some action (often of removal) employed to create a negative state or the absence of something. The difference is usually more obvious in the past participle. Usually the form with "un-" cannot even be used as a verb.
arm / disarm (remove weapons from)
unarmed - not carrying a weapon disarmed - having had one's weapon( s) taken away
infect / disinfect (remove possible sources of infection)
uninfected - not having an infection disinfected - having had possible sources of infection removed
qualify / disqualify (remove from competition or consideration)
unqualified - not having the proper qualities or qualifications disqualified - judged to be unqualified; having been removed from consideration
The prefix "de-" is also sometimes used in the sense of removal, forming verbs from nouns: "defrost", "delouse", "dethrone", "devein", "defrock", "declaw", "deice".
Sometimes the positive form has a prefix which is removed before the negative prefix is added. "encourage / discourage" "consonant / dissonant" With some dictionary work you should be able to discover the difference between the words in these groups as well. They are rather curious, not to say pathological, examples.
interested / disinterested / uninterested prove / disprove / improve integrate / disintegrate / segregate / aggregate assemble / disassemble / dissemble distinguish / distinguishable / indistinguishable distinguished / undistinguished claim / disclaim / unclaimed able / unable / disabled trust / distrust / trustworthy / untrustworthy cover / uncover / discover
"un-" is the native English prefix for negation, but it combines freely with nonnative roots as well. It is the most used prefix of its kind. It is used with verbs, adjectives, and adverbs, but also sometimes with abstract nouns -- not with concrete nouns ("*an unchair", "*an unbowl"). It can indicate simple negation (adjective) ("happy / unhappy") or it can indicate reversal of a process (verb) ("lock / unlock"). In this latter role, the prefix "de-" is sometimes used instead.
able / unable || tidy / untidy || cooperative / uncooperative || safe / unsafe || helpful / unhelpful || grateful / ungrateful || likeable / unlikeable || suitable / unsuitable || kind, unkind
Whenever there is a common word which is the opposite, the "un-" form does not exist: high / low (*unhigh, *unlow) fast / slow (*unfast, *unslow). But speakers sometimes mistakenly use such forms as "unthaw" for "thaw" (freeze / thaw, *unfreeze / *unthaw) or "unloosen" for "loosen" (tighten / loosen, *untighten, *unloosen).
pack / unpack || dress / undress || screw / unscrew || wind / unwind || tie / untie || roll / unroll || veil / unveil || cover / uncover
(Note how many of these form phrasal verbs with "up", e.g., dress up, wind up, tie up, roll up, cover up.)
code (encode) / decode || activate / deactivate || hydrate / dehydrate || humanize / dehumanize || escalate / de-escalate || brief / debrief
Here are a few curious examples. Get out your dictionaries! These could be challenging!
rail, derail; plane, deplane; attach, detach, unattached, detached, undetached; compose, decompose.
Does "derail" mean "remove the rails from"? If you have all planes removed from the runways, do you deplane the runways? Can you "rail" something? Can you "plane" something? What are the different implications of "attached" and "undetached"? Don't they mean the same thing (because two negatives (un, de) make a positive)? Is decomposing really the reversal of composing?