Hi Art,
I haven't had this happen to me personally. However I remember reading in the forums (and my memory may be hazy) that this would happen when Hal's voice recognition is active, so that it would "automatically press enter" after you spoke your sentence. People were complaining about it. Perhaps it functionally waits until input ceases for two seconds, assumes the user has stopped talking, and hits its internal "Enter." Try deactiviating the recognition and see if the problem goes away.
When using voice recognition, I prefer to use a third-party system, i.e. one that will let you dictate into a window. Hal just thinks you're typing text. Then you can either physically press enter, or use the voice recognition software's "press enter" voice command, to actually give Hal your sentence. This also (importantly) lets you edit any recognition errors before Hal starts learning your mistakes [

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Hope this helps!