Quit[]

<<FeynCalc`

R=GA[6];
L=GA[7];
yiPR=YiPR L+YiPRCC R;
ys1=Ys1 L+Ys1CC R;
ys2=Ys2 L+Ys2CC R;
SetMandelstam[s,t,u,p1,p2,-q1,-q2,mc1,mc2,ms1,ms2];

AmpSquare=Tr[yiPR.(GS[p1]-mc1 ID).ys1.(GS[p2-q2]+mb ID).ys2.(GS[p2]+mc2 ID)]//Simplify;

ID=1;Res1=AmpSquare

Clear[ID,Res2]
ID=GA[6]+GA[7];
Res2=AmpSquare

(Res1-Res2)//Simplify
%//DiracSimplify[#,DiracSubstitute67->True]&