One way can be

sed -e ' $q; /1$/!s/$/1/ ' file 

Or if there are no empty lines:

sed -e '$!s/[^1]$/&1/' 

One way can be

 sed -e '   s/1$/&/;t $!s/$/1/ ' file 

sed -e ' $q; /1$/!s/$/1/ ' file 

Or:

sed -e '$!{/1$/!s/$/1;}' 

Or if there are no empty lines:

sed -e '$!s/[^1]$/&1/'