I saved my recorded voice on an instrumental song.Now I want to continue recording,and while singing formed a new track,but when playing back the result the instrumental part gets double!how can I fix?thank you

Set the recording device setting in the device toolbar to the device that you are actually using (NOT “sound mapper” or “Stereo Mix” or “What U Hear”).
See also in the manual: http://manual.audacityteam.org/o/man/tutorial_recording_multi_track_overdubs.html

I tried to check the recording settings,but I could not fix.Can I record on the same vocal track that I saved without another track starts under the first?