I think that you’d explain the concept better accosting two words. For example lookalike replica. The word replica implies that the object is derived and shares most details with the original, the word lookalike would explain that the replica looks like the original, implying that it is not the same (otherwise the use of lookalike, a word that isn’t very common, wouldn’t be necessary).
If forced to use just one word, I would use lookalike (as stated previously by several people) or possibly use simile, which is actually the name of a figure of speech but that I think conveys the idea rather effectively (also it’s effective because it evokes the idea of something similar but the use of that word instead of the more common versions conveys meaning just like lookalike).
In general however I would aim to find different words to use in each situation according to the context, the intent of the person creating the replica in question and what you aim to transmit. You could describe much more with two words than just an adjective.
- An honest error while copying could be an incorrect copy.
- An ineffective tool similar to another would be a faulty replica.
In the examples you presented I would use:
The copies of texts transcribed by the monks were usually faulty because of inevitable human error.
Because faulty describes not the copies as a substantive but the action of copying so actually I’m using the adjective creatively by playing on that. Otherwise I would use tainted because usually in philology you consider the trickle of the copies from a source to what we have now (see Q source named after Quelle which literally means water source in German). Now that I think about it, imprecise also nails it quite nicely and by Occam’s razor could be the best choice.
Her lack of resources meant that her outfits were rip-offs of the ones she saw in fashion magazines.
Because the subject here is trying to copy, but I think the tone intended is derogatory (which is transmitted to me by the base idea that the copies contain errors). Also the word is often used in regards to design and outfits. In the opposite case I would be fine using copies/imitations or even just inspired/closely inspired if we want to express a certain legitimacy and appreciation in her action of copying the designs she observed (relying on the first part of the phrase to make the reader understand that her intention was to get a copy but she couldn’t to the original and not a novel creation).
The unicode characters '05AD' and '0596' are similar.
In this case similar works just fine. Other options could be confusable (maybe it’s me, but it sounds terrible) or easily confused or even easy to exchange by error. Of course the latter two are most effective if you’re speaking about the risk of committing errors due to their similarity. Similar here is the neutral choice, but the context is useful for picking the right word.