Avoid trying to move vignette sources twice

[flying-sheep]

Sometimes multiple vignettes can share source files. This breaks the file.remove call which expects unique arguments.

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the way i encountered this is that tools::pkgVignettes has two bugs:

1. it loops over all vignette engines (tools::loadVignetteBuilder results in a hardcoding sweave as last engine) and sets engine to the last one looped (=always sweave)
2. tools:::find_vignette_product is called with that engine and finds all r scripts in the directory prefixed with that name.

this results in duplicated source entries if a vignette’s file name is a prefix of another.

[hadley]

Could you please add a bullet to NEWS?

[jimhester]

Thanks! Merged in 72b1926