duplicate strings in French translation

This has been copy-pasted from #1155 (closed) since it is unrelated to the original issue. (one issue per issue only ;-) )

@speleo3 wrote to @marcjeanmougin : There are several different strings in po/fr.po which translate to "Superposition". Including "Screen" and "Overlay", which causes the "Duplicate option text" warning from the SimpleBlend filter.

Can you check if all these translations are correct? If yes, how should a french user distinguish "Screen" and "Overlay" blend modes?

Steps to reproduce:

User uses French as system language.

/Users/fstengel/Applications/Graphismes/Inkscape-ß1.app/Contents/MacOS/Inkscape -z -D testSVG-Fig.svg --export-file=testSVG-Fig.pdf --export-latex

What happened?

A warning appears.

** (org.inkscape.Inkscape:36755): WARNING **: 21:51:42.827: Duplicate option text ('Superposition') for parameter 'blendmode' in extension 'org.inkscape.effect.filter.SimpleBlend'.

What should have happened?

No warning.

Inkscape Version and Operating System:

• Inkscape Version: Inkscape 1.0beta1 (32d4812, 2019-09-19)
• Operating System: OSX 10.13.6

The issue is not OS specific.