Recently, I've been noticing that in casual speech people often unwittingly create new words from standard English words that are (a) of the same part of speech as the standard word and (b) meant to have precisely the same meaning. In particular, I've noticed that people tend to redundantly nominalize nouns or adverbialize adverbs. Some examples would be saying "seldomly" instead of "seldom" or "resiliency" instead of "reselience." I've looked up both of those words and have found that neither is new or uncommon, but my question still stands: What is it called when you make a new word from an old word and the new word means exactly the same thing as the old word?
Well, I have seen words on here for non-native English speakers which I have never heard of in all of my studies, reading, or education, so I rather expected someone to know this off the top of their head... I was not 100% joking in my comment -- I've always been curious.
It turns out that these are compound morphemes or other variants as described in quite readable English by Suzanne Kemmer on the Rice University website
I don't like to just leave a "link answer", but to copy-and-paste her entire writing here would be unethical and to summarize her summary would merely equate to the same.
Complexities of language aside, seems like "compound morpheme" is the best fit to answer your question.