De-dupe repl command history

[rtfeldman]

If I run the same command five times in a row, it appears in my history 5 times, meaning I now have to press the Up arrow six times to reach the previous unique command.

I don't think there's ever a situation where I want to press Up twice and have it show me the same command twice in a row, so it would be nice if repeat command entries were not added to the repl's command history.