Duplicate content

Could this be why my rankings have plummeted?

[Onders]

A couple of days ago a site that I set up a while back dropped from first page on G down to fourth. I haven't made any drastic changes but was wondering if I had been picked up on duplicate content. Basically if our main site (with backlinks) is [short domain].com which is abbreviated we also have the longer version [long domain].com which instead of being a gateway page is exactly the same as [short domain].com. Could Google be punishing me for this?
Although I have seen a few messages touch on this I'm still unsure of whether this is bad practice! Thanks

[edited by: ciml at 3:52 pm (utc) on Nov. 26, 2004]
[edit reason] Examplified [/edit]

[instinct]

You are more than likely facing a dupe content penalty.

Do a search on these boards for "301 redirect" for a solution to your problem.

[walkman]

"Do a search on these boards for "301 redirect" for a solution to your problem. "

does anyone have an timeable for 301 to take effect in serps from day of G finding it?