Well, considering that log lines ask you to compare your work to other already established works, I would say that there is little problem. I once had this great idea to write a story about David and Goliath from Goliath's perspective. I did a bunch of research, including how much a shekel weighs (Goliath's spearhead weighed 600 shekels, or 15 pounds, in case you were wondering) and then found out that there was another story out there that was very similar. Now, I still could have written it, but could I have sold it? I wouldn't have felt as good about it, because the idea wouldn't have been very original, but could I have done it in a way so that the execution was original? Probably, but I wasn't inspired anymore. The truth is, you could write a book and title it War and Peace, as long as you didn't copy Tolstoy's story. If you like what you have, finish it, even if it is similar to something else. Look at Eragon. It is very derivative. Terry Brooks wrote a book called "The Sword of Shannara" that was VERY similar to the Lord of the Rings, but it got published and was very popular. Does this help??