Repeated mapping generated from SIFTS

This corner case was spotted by [~smacgowan]. Sometimes repeated mappings are generated from SIFTS when the SIFTS record contains information for only one chain in a multi-chain PDB entry

To reproduce:
   A. Load sequence below
>SNRPA_HUMAN/12-83
----------------------------------------------IYINN-L--------N-------E--
-----K-----I-----K--------K----D--------E----------L--------K--------Ksl
--yaI------F-------------S---------Q----F-----------G----------Q--------
---I-----L-----------D-------------I------------L------------V----------
--S-----------R-----------S---------------------------------L-----------
---K-----------M----------R-----------G---------------Q------------A----
-----------F------------V-----------I-------F--------K----E--------V----
-------S-------S--------A-----T--N-------A----L----R----S----M----QG--FP
F-YDKPM-r-----------------------------------------------------

  b. View structure for 3ucu
  c. View mapping output

Only one mapping is expected in the output but three repeated mapping is generated for all the three chains in the PDB file entry