This Week in Relevance

You should try your best to ensure that this does not happen, and here's why:

You will not be bidding against yourself so much as you will not have a good sense of which campaign/ad group your ad will serve out of. Those 2 ads may deliver significantly different user experiences with different conversion profiles. Google will select which ad to serve based not on conversion value delivered, but on AdRank (Quality Score*Bid), which would potentially be suboptimal for your business because it does not take your back end metrics into account.  

Also, if those keywords live in different ad groups within the account, you cannot take advantage of Google's ad rotation and optimization capabilities so if you wanted to actively test the performance of different ads against the same keyword you would be much better served running multiple ads against that keyword in the same account.

The more fine grained control of an account you have as a manager tends to drive better and more comprehensible results as you leave less up to Google to make decisions on what ad to serve when.

To the question on keyword elements, you should have no issue when having different keyword elements in different areas within the campaigns unless you have poor coverage and the keywords are on broad match, thus forcing Google to decide which ad to serve in the cases where a query has that element but you don't have an actual keyword that matches it precisely.

In Google AdWords, to what degree should you allow ad groups to share keywords, or keyword elements?