@Emily_Ensembl: I actually believe otherwise. In Ensembl, TAF9 has two ENSG IDs: ENSG00000085231, ENSG00000273841; while on HGNC, they have one ID for each gene:
AK6: ENSG00000085231; Entrez:102157402; HGNC:49151; Link: http://www.genenames.org/cgi-bin/gene_symbol_report?hgnc_id=HGNC:49151
TAF9: ENSG00000273841; Entrez: 6880; HGNC:11542; Link: http://www.genenames.org/cgi-bin/gene_symbol_report?hgnc_id=HGNC:11542
So something happened on Ensembl side!
I think it has something to do with mapping interval/coordinates back to gene names.