Consecutive injections are causing ghost peaks in my GC chromatogram, how can I avoid this?

There are two common causes for this phenomenon. The first is that the sample is being contaminated with every injection.

This is common when a needle pierces the septum of the vial and deposits a small portion of the septum into the sample. The septum dissolves over time and creates more ghost peaks.  

A second cause is that sample components are not getting removed completely with each injection and are carrying over to subsequent injections.

If this is the case you will generally have peaks eluting near the end of the run that will likely increase in frequency and intensity from injection to injection.

To avoid compound carryover, increase the final temperature used at the end of the temperature program if possible within the limits of the column and/or extending the run at the end to better clean the column between injections.