I need some clarifiaction on getting double aptitudes

f.e.

I get BS as aptitude from sharpshooter training and from some of the specialities(Heavy gunner, Medic ….)

so when i get these is it right that the char get for the double only the Finesse aptitude ,or could i choose from some other one´s ?

As far as I am aware you can choose any aptitude, that shares his name with a characteristic, so when you get Ballistic Skill two times, you can choose any other characteristic aptitude like willower, toughness, strength etc. If this is wrong, please somebody correct me.

You are correct. However, I'd suggest that you should also be able to take the non-attribute aptitude that applies to the attribute you are switching out. For example, a Heavy Gunner whose regiment grants Ballistic Skill (meaning they're renowned for their precision) should IMO be able to switch that for Finesse to improve his BS advances further.