Can't export .otf with Fontlab

I'm trying to export a personal cut of Lettera but Fontlab doesn't want to cooperate. At the end of the exporting process it says:

[FATAL] cmap{plat=3,script=1,lang=0}: multiple glyphs Ugrave _244 mapped to code
ERROR: There was a problem while compiling OpenType font. Final .otf font is not saved. Please, check OpenType features definition for errors

I hope someone could help me.

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