First off, do you have iTunes latest update...11.0.5? If not, get it updated first.
Did you do this?
"This update addresses an issue with iTunes in the Cloud, where some purchases may download or play unexpected items. If you have any devices running previous versions of iOS 7 beta, you must:
1. Install iOS 7 beta 6
2. Install the ResetMusicAndVideosLibraries configuration profile
3. From Settings > Music, tap the Reset Media Library button
4. Restart the device
"
If not, do above first, if yes, then, (I have had issues w/syncing music from time to time with the iOS 7 betas), (assuming you're connecting via USB cable to computer)
try turning off syncing for music, quit itunes, disconnect and power cycle the phone,
then connect and do a sync (with syncing for music still off), quit itunes, disconnect phone,
now connect again, open itunes and turn syncing for music on, do a sync, (wait for it to complete). Disconnect, power cycle phone/quit itunes,
then connect it all again and sync again, if it worked correctly the last sync, it shouldn't be syncing more music. I find things to be a bit quirky with itunes & ios 7 still, but, if you do all of this once, it should clear things up, if not, then,
(I interrupted music syncing the first time I put ios 7 on and it kept doing oddball things, not all my music was there, duplicates of some songs, etc.)
I have less than 1000 songs that I sync to iPhone @ 128K (I use pandora a lot, and now itunes Radio, so I don't need my entire library on the phone), and it's been better since doing the above reset, but, some that have extensive music libraries are still having issues...how many songs do you sync w/iphone?