fix for unmapped xref having possible multiple masters having same index
fix for unmapped xref having possible multiple masters having same index

at7/ensembl,willmclaren/ensembl,muffato/ensembl,at7/ensembl,danstaines/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,Ensembl/ensembl,Ensembl/ensembl,Ensembl/ensembl,mjg17/ensembl,muffato/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,mjg17/ensembl,at7/ensembl,muffato/ensembl,willmclaren/ensembl,muffato/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,danstaines/ensembl,at7/ensembl