The Bash shell completion for pacat --device somehow combines the names of sinks and their monitor sources, resulting in mess like the following:

> pacat --device=alsa_<TAB>
alsa_input.pci-0000_00_1b.0.analog-stereo
alsa_output.pci-0000_00_1b.0.analog-stereoalsa_output.pci-0000_00_1b.0.analog-stereo.monitor

Note the concatenated string containing a sink name and the sink's monitor source name. Here's a snippet from _pacat():

_pacat () {
    ...
    case $cur in
        ...
        --device=*)
            cur=${cur#*=}
            comps=$(__sinks)
            comps+=$(__sources)
            COMPREPLY=($(compgen -W '${comps[*]}' -- "$cur"))
            ;;

The bug is probably in the _pacat() function, because the __sinks() and __sources() functions seem good. I suspect the COMPREPLY line is incorrect, but I don't know in what way.