Whats the minimum length for a clip to make transitions work?

[Crystal24149397jok1]

I'm trying to add a cross disolve to two clips that are about 40 seconds each in length. I thought that would be enough frames for the transition to work, but I keep getting a warning that there aren't enough frames and that premiere pro will have to double frames to make the transition work. 
I'm not sure why I can't put a 1 or 2 second transition in between two 40 second clips but if that's that case, what is the minimum lenght a clip should be for the cross disolve effect to work properly without doubling frames?

[Ann Bens · Accepted solution]

You are probably put the very end of a clip to the very beginning of a clip. That will give repeated frames.

You need to trim the clips back first. Clips need handles in order for transitions to work properly.

If the cross dissolve is 1 second and your framerate is 50 fps, both clips need 25 frames of handles to put the transition in the middle.

Applying transitions in Premiere Pro - handles

[Crystal24149397jok1]

That fixed my problem!
Thanks!