Due to the nature of the TsigSecret in the Server{} object, it's not possible to re-use TSIG names across different zones.

For example:

TsigSecret = map[string]string{"axfr.": "so6ZGir4GPAqINNh9U5c3A=="}

Here the tsig name is axfr. which means that if any other zone wanted to use axfr. as well, it would be overwrite this one. Alternatively, the server could specify the TsigSecret to use zoneName for the TSIG name, which is better because duplicates would not occur, however this means that every client using that zone would need to have the same TSIG and also wouldn't allow TSIGs to be re-used across multiple zones which again is not great.