Unfortunately that is not deterministic. There is no guarantee that the "last record" would win, among other things because there is no such thing as physical ordering in SQL. Books online deals with it in this section of the UPDATE documentation:
Use caution when specifying the FROM clause to provide the criteria for the update operation. The results of an UPDATE statement are undefined if the statement includes a FROM clause that is not specified in such a way that only one value is available for each column occurrence that is updated, that is if the UPDATE statement is not deterministic. For example, in the UPDATE statement in the following script, both rows in Table1 meet the qualifications of the FROM clause in the UPDATE statement; but it is undefined which row from Table1 is used to update the row in Table2.
http://msdn.microsoft.com/en-us/library/ms177523.aspx
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Your lack of planning does not constitute an emergency on my part...unless you're my manager...or a director and above...or a really loud-spoken end-user..All right - what was my emergency again?