Itunes

For some odd reason, I have multiple copies of songs on iTunes. When I go to delete a copy, one copy is listed as 1.m4a and the other is listed as .m4a. For consistency, I deleted songs with 1.m4a. When I return to iTunes, I found out that a lot of the songs have been totally deleted so I have to go to my external to retrieve songs. How do I know which one to delete?