animation loop

I made an animation of 40 frames, with keyframes 1, 11, 21 and 31.
From frame 31 onwards the animation freezes until frame 1.
How can I have an interpolation between frame 31 and frame 1 (virtual 41)?

Make a copy of the keyframe at frame 1 and move it to frame 41.

How do you make a copy of a key frame?

-> Open an Ipo Editor window
-> Press K
-> Select one the vertical lines
-> Shift-D
-> Move the key
-> LMB to insert.

Keep Ctrl pressed to lock to frames.

Many Thanks SoylentGreen.

I used 2 objects for this animation. Each one is animated differently.

For some reason one object does not move from frame 31 to frame 41.

And the other object is has been rotated slightly at frame 41 compared with frame 1.