What do you call mRNAs that translate to the same protein?
This is by no means a consensus, but this paper from 2000 proposes a new term: Isotranscript.

According to the authors:


  Duplicated genes often encode distinct proteins that differ by only a few amino acids, such as the two mammalian S27 isoforms, S271 and S272, documented here and the yeast and Arabidopsis S27 isoforms identified previously. On the other hand, we have found that different S27 transcripts encode 100% identical proteins. This phenomenon has also been described previously for the histone subtype, H3.3, in which two different transcripts encode the same H3.3 amino acid sequence . We have chosen the terminology “isotranscripts” to describe such mRNAs. (emphasis mine)




Source: Thomas, E., Alvarez, C. and Sutcliffe, J. (2000). Evolutionarily Distinct Classes of S27 Ribosomal Proteins with Differential mRNA Expression in Rat Hypothalamus. Journal of Neurochemistry, 74(6), pp.2259-2267.