iTunes changes file names of already in-the library songs and then can't find them

OK. This problem I've been having for sometime on different Mac OS's and hardwares. iTunes changes the file names (adds a 1 to the end of the file name) of certain songs that have been in the library for some time. It then cannot find the song. I have to either link the file with the new file name, which is always in the same folder, with the song through locate file option or restore the file name to its original by deleting the 1 appended to the end. Usually the same songs get changed, the period being randomly. Any clues any one?

Also when importing some songs by adding them to the Add to iTunes folder, iTunes adds many entries in the library linked to the same file, usually without being able to move the file out of the Add to iTunes folder. The round about I found is to import files directly. Any help would here would also be appreciated.