I've come across this in multiple grammars: Two grammatical cases (e.g. ergative and instrumental) are said to be "homophonous" - they make use of the (apparently?) same marker and yet, they are annotated differently in the examples. What are the factors through which can be decided whether those cases are justifiably labelled distinctly or if I should view them as one, regarding the "difference" between the two simply as "additional semantic information"?
Here are two examples for the ergative/instrumental distinction that I had at hand, there are probably better ones:
In the grammar for Ese Ejja (ese, Vuillermet 2012), the author repeatedly mentions the "homophonous instrumental" (p. 197):
When the ergative (or homophonous instrumental) case is associated with nouns, then only (gliding) transitional sounds is occasionally heard with the front vowels i and e and with the back vowel o – cf. the missionary transcription for Ernesto + =a .
In the Bantawa grammar (bap, Doornenbal 2009), "instrumental" is merely called a "role" that ergative might take (i.e. a possible translation, p.75):
The ergative may also be affixed to nominals in roles that would usually be called ‘instrumental’. Noun phrases that are marked with the ergative but do not act as the agent in a verb frame translate as instrumental modifiers. There is no separate instrumental case, however. Ergative-marked instrumental phrases are adverbial modifiers denoting either the cause, the method or the instrument of the action.