[xavier loebl]"so I have my 25fps footage with doubled frames"
You don't want doubled frames -- you want motion estimation to calculate new frames in between the existing ones.
Use Time Remapping (making sure that Layer > Frame Blending > Pixel Motion on), Timewarp, or Twixtor to create a slo-mo version of your clip that runs at 50% of the speed of the original.
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