In all honesty, I'm sure that you are all honest translators, and that you would not knowingly deceive a client. Nevertheless, there are always situations that keep us doubting as to what we should do in certain situations. Let me give you a few of them:
The document is already in the target language (in my case, English). You figure out if not immediately, somewhere down the line before sending it off to the end client. All you have to do is tell them that they sent you the wrong file because it is the target language, and they will thank you, and you hope that they will send you more work soon. I just hope that it is not a 100,000-word job where you could have greatly profited from it.
One or several pages of the document is a duplicate. This situation is a bit more subtle, especially when clients don't proofread translations, because even with a cursory glance, it could be looked over. But then again, maybe it doesn't go undetected, and how would that make you look? If this happens, you should translate only one of the duplicate pages, but it is also important to send the client a note about this, because if the translation turns out to have fewer pages than the original, this will also be confusing. The good thing is that a duplicate page does not subtract too much from the total fee, and also the client will no doubt be thankful that you did spot this.
The client sends you a document to be translated that you had already translated for them before. This may be a situation where you could completely get away with retrieving the earlier translation from your hard disk and attaching it to an email and hitting send. However, you would be cheating them out of their money because you didn't put any effort into the work, and you are in essence holding back information by not telling them. Again, I hope this isn't a large document, because it would really hurt to know that you wouldn't be earning all that cash!
Now, I have to admit that this scenario has never occurred in my translating career, but it very well might. The client sends you a document whose translation is already on the Internet word for word. It would be so easy to retrieve it and hit send and collect your fee. No one will be the wiser, but I think it would be a win-win situation to at least let them know that the translation is already out there, or you could even send the link to the document so they can decide for themselves. If they realize how honest you are, they might be so thankful that they will send you more work, thus cementing your business relationship with them.
And that's the honest truth!