Try the following:

alias +test "echo TEST"
alias -test "echo test..."
bind Q +test

Now hold Q for a few seconds.
TEST
test...
<short pause>
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
TEST
test...
et cetera.
It seems to me like there should be a single TEST/test... pair.
-:sigma.SB

Not a bug. It's just the way X works.
The Windows version does multiple TEST and then 1 test...

X sends key down and key up messages for key repeats.