I'm a brand new GM to both Dark Hersey and RPG games. I just ran a short little sample campaign of Dark Heresy a little while ago, and I ran into a problem with the Deja Vu power. Here's the scenario that lent me this trouble.

The players were clearing out a mine, and ran into a group of mutants. They killed all but one, and it got into melee with the Guardsman player. It swung it's weapon at the player. When the psyker's turn came up, he used Deja Vu on the mutant. Then the Guardsman moved out of melee with the mutant. When it came to the mutant's turn, I had it swing into clean air.

Is this how the power is supposed to run? Reading over it in the book makes me believe it works this way, but after watching my players basically daisy-chain the mutant around made the power seem a little broken. So, I was wanting to make sure this how it works before my next session.

It works exactly how you described it. Just remember about rolling for the affected one to resist this power, as severity of this test may vary depending on situation - battle frenzied mutant or badly wounded one should get bonuses to the test, especially when it's a matter of life and death for the antagonist. Don't be afraid to give +20 or even +30 to the WP test of the antagonist if you deem the situation demands it.

But if the WP test is failed, then your players can do as they did. One thing - I guess it affects the antagonist in its turn, so a PC, unless disengaging, cannot walk away from the mutant because the mutant would still gain the extra attack (I'll repeat - unless he disengaged). And there is one more thing - remember that the antagonist still makes a swing or shoots, which can have unpleasant conequences for anybody in proximity. Keep that in mind. An accidental bullet finding their way to one of the players (or an axe swing which is so broad that it touches a PC) will keep them careful with using the power.

Also keep in mind that the more the psyker abuses his powers the higher chance there is of perils....which can be party ending....and also funny for the GM... If it starts to be to much of a problem then try and rope them into a dark pact.....which for most players the promise of more power is always a YES!