I'm not sure if this is a bug, but it seems like one. if I run something through this trigger, it gets passed through OnPluginScreendraw twice, but only shows up in the output once as it should:
<triggers>
<trigger
enabled="y"
match="test: *"
omit_from_output="y"
send_to="2"
sequence="100"
>
<send>%1</send>
</trigger>
</triggers>
<triggers>
<trigger
enabled="y"
match="test: *"
omit_from_output="y"
send_to="2"
sequence="100"
>
<send>%1</send>
</trigger>
</triggers>