package fails to build twice in a row

step 1: unpack the hirsute source somewhere and cd there
step 2: debuild (success)
step 3: debuild (failure)

$ debuild
[...]
make[1]: Entering directory '/tmp/squid-deb-proxy-0.8.15build1'
dh_auto_build override_dh_auto_build
        make -j4 "INSTALL=install --strip-program=true"
make[2]: Entering directory '/tmp/squid-deb-proxy-0.8.15build1'
echo "nothing to do"
nothing to do
make[2]: Leaving directory '/tmp/squid-deb-proxy-0.8.15build1'
# use hte right default mirrors file for the distro
ln -s mirror-dstdomain.acl.Ubuntu mirror-dstdomain.acl
ln: failed to create symbolic link 'mirror-dstdomain.acl': File exists

mirror-dstdomain.acl needs to be cleaned before the build. Alternatively, use -f for the ln command.