If you have not as many different video clips as you wish you will need to use the same clips over and over again. The distribution of clips can be managed with Repeat and Distance settings.

But even with the best Repeat and Distance arrangement the same video fragment repeated in different mosaic places may cause unwanted visual patterns that will make obvious your shortage of video footage.

Add Random Video Offsets option from Edit menu may help to avoid it. It will randomly moves frame pointers for same clips for a small time range. After that your clips will not be so visible synchronous.

From the other hand this may cause worse overall mosaic. You need to find a compromise between better visible main mosaic theme and visible visual patterns.

You always can remove these offsets by selecting Remove Random Video Offsets from Edit menu.