You may find in your literature review, or as you monitor new literature, that someone has already addressed your topic.

These items give reasons why it is important to search thoroughly and also monitor new literature:

• Ph.D. Comics 
• Mulitple Discovery (Wikipedia)
• Two of a Kind: Multiple Discovery AKA Simultaneous Invention is the Rule
• Re-inventing Willis M.V.Simkin, V.P.Roychowdhury

 What do you do if someone has published before you?  Here are some articles with tips:

• Sometimes you’re the scooper, and sometimes you get scooped: How to turn both into something good
• It’s not the end of the world if your research gets ‘scooped’
• How to deal with being “scooped”: The vast majority of science is a process of derivative, incremental advance.
• What happens if someone publishes a paper on your current PhD thesis topic