from the article: "...predicts Mithril’s average investment will be north of $20 million"
That doesn't sound like overlapping mandates to me. A more interesting question would be around the (potentially confusing) signalling issues if Founders Fund didn't follow-on in a later investment but Mithril did come in.
On the other hand, it allows Thiel to stay involved in every stage of fundraising, coming in and leaving as early and as late as he wants. Really, sounds like a good way to hedge his bets, and open up a new arena for him.
That doesn't sound like overlapping mandates to me. A more interesting question would be around the (potentially confusing) signalling issues if Founders Fund didn't follow-on in a later investment but Mithril did come in.