Race condition on incremental backups
I want to know if there is a problem to occasionally start a new incremental backup when the previous one is still running


For SQL Server 2000

You would not be allowed to start new differential backup unless the previous one finishes.

For SQL Server 2005 and above

You can start new differential backup while old one is still running but new one would be blocked by already running differential backup. The new diff backup will only start after old one has finished.

This Blogs.msdn article and this SE thread would help you in understanding similar behavior