ensure modifications with same `vendor_code` are not duplicated

[dave-doty]

I created a design (don't know how, need some research to reproduce) in which two strands had a 5' modification with the same IDT text (/5Biosg/) but with different keys in the Map Design.modifications_in_design. One had a prefix "5'-" and the other did not.

This was from the 2-helix example; perhaps the problem is the 5' biotin that's already there? It could have an outdated key in the design somehow.

[dave-doty]

Partially addressed by PR #927 (changed example design with biotin to use the key name that's used when adding mods in the web interface), but there's still a way to get inconsistency with mod keys, so leaving this issue here until we address this somehow.