We should have really seen this coming. before Marvel Studio's was formed, the rights to alot of the companies comic book characters were shopped out to different studios. While many of those rights have returned to the fold, mainly due to disappointing movies (looking at you Ghost Rider and Punisher), the one big holdout is Fox, and, specifically to this article, X-Men. A rights grey area exists between Marvel and Fox when it comes to two characters important to both the X-Men and Avengers franchise: Quicksilver and The Scarlet Witch. The son and daughter of the X-Men's nemesis Magneto, they are also important and long standing members of The Avengers, and as such the rights belong to both Fox and Marvel, with the condition being if Fox uses them their history with The Avengers cannot be mentioned, while Marvel cannot go near their lineage or refer to them as mutants. Now, it looks like we will be getting two versions of the characters with both studios making known that they plan to use the characters in both X-Men: Days of Future Past, and The Avengers 2.
It was earlier revealed that Joss Whedon had written a draft of The Avengers 2 in which both characters appear (with Saorise Ronan used as the 'model' for The Scarlet Witch), with many people believing Fox wouldn't let this slide. Which they didn't. A tweet appeared on Bryan Singer's Twitter feed welcoming American Horror Story's Evan Peters on board the currently in production X-Men: Days Of Future Past as Quicksilver. While initial reactions were of a inter company crossover, brought on by Singer's tweet reading "Before he was an Avenger, he was just a really fast kid"a report from Hitfix put paid to this, saying Quicksilver is being included in the X-Men sequel as part of an action scene Singer envisioned, making use of the characters super speed, with no eye of connecting to The Avengers 2. I wouldn't be surprised if this went further and The Scarlet Witch will make an appearance in future X-Men sequels.
So, at the moment, this means we are getting two versions of the same character in different franchise, from different studios. Someone will have to give, and relinquish use of the name Quicksilver, referring to by his given name Pietro, otherwise it is going to get confusing. It's going to be interesting to follow these developments, and I am waiting for Marvel Studios to react to this announcement.