If you want the final output file to have no duplicate/repeated frames, then you will have to create a file with a frame rate of 18fps. This is possible, but uncommon. So you will have to use custom settings. Here is my suggested process:
-
Create a custom video mode (Settings->Video Mode->Custome->Add…) Make sure the “Frames/sec” field is set to 18.
-
Set the video mode to your new custom mode.
-
Open the file you want to convert.
-
In the Properties panel, set the speed to 0.6x.
-
Encode the project - make sure the “Frames/sec” field is set to 18.000.
If you follow these steps, the resulting file will play back at the proper speed and there will be no duplicated frames.