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Would it be possible to make single frames into double frames without changing the length of a layer? - other than deleting frames by hand -
If yes, can somebody please tell me how to do this?
Thanks. Conny.

if I understand your question correctly, here's what I would do :

supposing your animlayer is 200 frames long, drag-in the end-handle to shorten its length to 100 frames
(the nb of frames of the current layer is diplayed in the title-bar of the timeline window), "decrease layer length" set to Shrink mode, no interpolation (Interpolate gadget =Off), Apply,
the layer is now half the length of the original, then select all frames (double-click under the frames thumbnails),
and Layer/Add Exposure, (or hit the [+1] button of the Instance panel).

Yes, great.
Thank you very much!
With video I use 'speed' conversions in Final Cut Pro and with this photomatererial I couldn't think of an equivalent in TVP -

you can also shrink to half-length as mentionned above,
then modify the frame-rate of your Project to 12.5 if your source was 25 fr/s,
to 15 if it was at 30 fr/s.