Alright.
This depends on the properties of Jack's dagger.
If Jack's dagger says something something outright as:
...Then yes, you get +1 Strength from the Dagger, so long as the item is equipped or on Jack's person. More hilariously, if Jack's own dagger was being stabbed into him, he'd retain the +1 Strength, and the stabber wouldn't without permission.
Let's tweak this a bit, something you would typically see more in custom weapons:
---At this point, especially with items that mention the character's name, it would probably mean that unless overridden by being transferred over to someone else with the character's consent, if the consent was not given, then a character would just be given a dagger that only gives Jack +1 Strength. However, Jack would need to be holding onto the Dagger, of course, if he wanted said +1 Strength.
Basically, a repeat of the above.
Unless the Dagger specifies that it must be attacked with in order to gain the +1 Strength, then yes. Obviously, if you wish to only give yourself the +1 Strength when attacking the dagger, that would be considered good RP ethics. Both you and the person you're fighting should definetely come to terms with how you are both handling your items, as a precaution.
Ultimately, this boils down to you and the person you are RPing with. If its done from a RPing perspective, a person can take the knife and stab at you, because you are allowed to do the same in kind. However, whether the item grants him the +1 Strength is up to you, although it could be considered sportsman like if you allowed it thus.
However, as previously addressed, we do not solicit items being stolen on here. Not unless express and mutual permission is granted, for the most part items cannot be stolen. There has never really been a case where an item has been stolen except for RP storyline purposes.
You might recall one of the rules that states that once you transfer an item to someone else, you may not receive that item again. Its gone from your inventory forever.
This was a precautionary rule established back in the days of 552 when people were transferring items back and forth as basically free EXP and was highly abused.
...However, because of this rule, we can read thus:
-To transfer an item (i.e. with OOC consent, which overrides the IC non-conense, as IC-wise our characters feel differently from us), there must be mutual consent between both the giving and the receiving party.
-Griefing is illegal.
I hope that satisfies the subject so far. Though, if this arises further questions, please feel free.
Again, keep in mind that Leon may step in at any time, in the event that I have addressed anything incorrectly.