Parallel downloads with duplicated files in mk

Parallel downloads were introduced in 45c00abe to speed up RTD build (!25 (merged)).

Unfortunately, when more than 1 core is used and at least one of the file is duplicated in the list of missing kernels (within the number of core window), the 2 thread will compete and the second one will generate an FileExistsError.

This situation appended for JUICE with the mk juice_crema_5_0.tm where juice_mga_crema_5_0_default_f20160326_v01.bc is duplicated l.96 and l.99 in SKD v310 and v320.

This issue was reported to the ESA Spice service on July 27th 2022 and should be addressed in the release.

Possible fix:

Ensure that the download list missing in the MetaKernel.check() function contains a unique list of output files.

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