In this Cracked article http://www.cracked.com/article_19852_5-famous-movies-that-shamelessly-ripped-off-obscure-ones.html it states that Ridley Scott ripped off two other lesser know movies to make Alien. However according to the wiki http://en.wikipedia.org/wiki/Alien_%28film%29 Dan O'Bannon takes the credit for stealing from these two movies. So how much influence is from the script and how much is from the director? Also even though it's a comedy site they don't actually say that the director never gave credit to these two movies so has Ridley Scott ever acknowledged these movies as influencing his designs for this movie?
Cracked.com is a great place to find false information. I wouldn't recommend taking them too seriously, but that article is correct about Alien.
"Good artists copy. Great artists steal."
Dan O'Bannon explicitly stated in an essay he stole elements from those movies (it's available on the Alien special features disc). So it is really the script that contains stolen ideas, not the directing. I don't really like using the word "stole" for this, because it seems to me that parts of the script were "inspired" by those movies. Nothing can be completely original--writers are always influenced by others. Dan O'Bannon seemed like a nice, laid back guy, and he intended to create a work of art by writing the script.
With the combined creative genius of Ridley Scott's visionary style, Giger's alien designs, and O'Bannon's script, they were able to create something truly unique, even if it did use some ideas from other films. I would highly recommend watching the Alien special features if you want the full story.