The "U" in URL stands for "Unique" (some say Uniform) which should be a good clue here :)
Best case scenario is that you split any "link popularity" between 2 different URLs. Imagine half the visitors to the page using one URL and half using the other URL. Each URL only gets 50% of the traffic, 50% of any popularity "score" that Google might apply. To that end, it makes sense to decide which URL is the one you want to be the Unique reference for the document and redirect the other one (using a 301 redirect).
Worst (or worse) case - you run the risk of a duplicate content penalty or Google simply ignoring one of the URLs in favour of the other but you're not able to pick which one.
If you don't wish to set up a 301 redirect then you should make use of a "canonical url" tag to tell Google (and anything else) which is the source url. This will normally then be the one that is indexed.
Find out more about Canonical URL tags at https://support.google.com/webmasters/answer/139066?hl=en