If it's something like the H4N recorder, I usually set the recorder to mono if I know I'm only feeding in one mic.
Alternately I would set Premiere to add stereo tracks as mono. Edit > Preferences > Audio, change stereo from "Use File" to "Mono" and you can set that for 5.1 and other multitrack audio. This splits each channel into individual mono files.
If you have a huge mix of mono-as-stereo files, I would just downconvert them in Audition before bringing them into Premiere so they are true mono.
Angelo Lorenzo
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