Yes, especially when you are just starting out and writing your first screenplay.
I actually came across a movie a month or so ago on Netflix that had a description that seemed similar to what I am trying to write so I went ahead and got it. Upon viewing it, I realized that, much to my surprise, it was very similar to what I am trying to write. So similar in basic plot, the non-linear format to even how the title character is introduced through other people it was surreal.
Needless to say, I was bummed out. But after a day or so of being bummed out about it, I re-watched it and took notes on how my screenplay is different, what I liked and didn't like about it and how I could change the similarities into what I think is better and works for my script and its world, etc..
The best part of it all is that I am still writing because what I learned out of it was a) the story has already been told so I can just focus on tightening my own script that while may be similar is completely original, b) it's already been made which I think can't hurt as it shows that there was interest and it might improve my chances of selling or directing my script, and c) it brought in some decent names (i.e. Toni Collette, Brittany Murphy, James Franco, Giovanni Ribisi, etc.), so I think that something like this may be able to get some interest from the industry since its a similar project.
But then again, I am just a first time screenplay writer that has wildly ambitious thoughts and dreams and doesn't know how the industry really works.
And if anyone is interested the movie is called, The Dead Girl.