The Loup-Garou's vulnerability to silver can be tagged as many times as you have fate points because it's part of the creatures high concept. It won't go away.
This wouldn't be tagging or invoking an aspect, it would be satisfying "The Catch." Tagging is different. (as is invoking)
Tagging and Invoking seems to be one of the areas that confuses people the most, so here's the short version of how I understand it.
Tagging (two ways to do it):
Assessments: You run into an Ogre Thug with the aspect "BIG BRUTE." You've never run into him before, but the GM states "An Ogre comes thundering in smashing through the wall shouting 'Crush puny human bugs'." Being the savvy player that you are you look to the GM and say, "I'm pretty alert and aware, is there anything I particularly notice about the Ogre as he's coming at us?" The GM glances at the sheet for the Ogre and decides that an assessment using alertness makes sense, and tells you it would require Great Alertness. You roll the dice and total +5, the GM tells you "As he comes smashing through the walls, you notice that this Ogre is a pretty BIG BRUTE." You get to go first in the round of combat and look at the GM telling him, "I know this guys is a BIG BRUTE, so I'm going to tag that to distract him while I go for his knees." The GM nods and you roll your attack, adding +2 to the roll to reflect the Ogre not paying attention because you called him a mean name.
Maneuvers (or other added aspects): The Ogre is now pretty pissed, and he punches you, causing quite a bit of pain. Thinking fast, you decide to change tactics and tell the GM, "I'm not going to attack this time, instead I'm going to make a maneuver to throw dirt in his eyes." The GM nods and tells you to roll weapons. You roll a +4, but the Ogre only rolls a +2 on his defense. You apply the temporary aspect "DIRT IN HIS EYES" to the Ogre. The Ogre is still pretty pissed, so it lunges at you. Thinking about what you just did you decide that survival is probably important. You turn to your GM and say, "He's got a bunch of DIRT IN HIS EYES, so I'm going to tag that while I dodge to the side". The GM nods, and you add +2 to the athletics roll to get out of the way, this time escaping the Ogre's clutches.
In both cases, you only get a tag the first time you discover an aspect, or the first time you add an aspect to him.
Invoking: Later on you are fighting the same Ogre as before, still with "BIG BRUTE". The Ogre takes a swing at you, and you duck under it. You roll athletics to defend, and tell your GM, "I'm going to take advantage of the fact that he's a BIG BRUTE and duck under his legs to escape." Your GM nods and you hand over a fate point, and receive +2 on your roll as a result. You spend a fate point, you get a +2, and you have Invoked the aspect. You can do this again the next time he swings at you, as long as you have fate points.
I realize both of those are long winded, but I felt that an example would be helpful in clearing up the confusion. As for the "repeat tagging" that's up to a GM. Tags should be used to encourage players to use maneuvers and do their research, not as a tool for players to just stack tons of pluses on every action. In terms of my examples, the first player assess that the Ogre is a BIG BRUTE and gets handed a "tag token" he can use to activate that aspect. That player then turns to his friend and says "The Ogre's a BIG BRUTE, get him," and hands his friend the "tag token". Player A can no longer use this tag, but player B can. If both players discover the aspect independently (for example, Player A assesses it with Alertness but doesn't tell anyone, Player B assesses it with Lore [knowing that Ogres tend to be brutes] it becomes a question for GMs, but I'd allow both of them to tag it in order to reward and encourage characters with high investigation, Scholarship, Alertness etc.)
