-std=c++14 after -std=c++11 in my build command, which wins? [duplicate]

I have both options on in my Makefile, Basically, my Make file looks like this

$OPT_FLAG := -O2 -std=c++11 $CXXFLAG += $(OPT_FLAG) -std=c++14

so actual build command is sth like

g++ -m64 ... -std=c++11, -std=c++14 ...

and of course $CXXFLAG is passed as argument to invoke gcc build. In this case, am I using c++14 or c++11? my gcc version is 5.4