The rules state that a player cannot choose to take a condition when he already has one of that type.
How do you deal with cases when an event forces you to draw a condition card?
As an example, let's say my character has a debt card and he is asked by an event in the game to draw one. Do you
The very detailed reference guide covers this case. From p.4, "Conditions":
An investigator cannot have multiple copies of the same Condition. If he would gain a Condition that he already has a copy of, he does not gain another copy of that Condition.
ire_and_curses's answer is correct, you can not gain multiple copies of the same condition. There are, however, two caveats to this that are important to note.
Curses and Blessings have their own rules for handling multiple copies. If you were to gain a second one of either, you flip them instead. This is printed on those cards and, as always, text on cards overrules general rules.
In the future more conditions with special handling for these cases may be added.
This rule means that you can not "pay" for an effect if you already have the condition required. This means that when you are instructed to "gain a specific condition" or something else (worse) happens, you have to resolve the "something else" if you already have that condition.
Same goes for text like "you may gain a specific condition to gain something 'good'", you can't pay for the good stuff if you already have the condition.
