The rules states that you cannot have two of the same conditions. You can take a debt condition to gain two additional successes on your influence test to gain assets from the reserve. Thus if you already have a debt, does that mean that you automatically gain two additional successes for each gain asset actions?
If you have ANY condition (whether it be debt, or a dark pact, or hallucinations), you cannot gain it again while it is in play. So, if you take a debt (getting 2 additional "bucks" or influence for that purchase), on any subsequent turn while you are still in debt you cannot do that again or get the bonus. It only happens once.
If, on a later turn, you are able to rid yourself of your debt (e.g., through its reckoning event), you can take on another debt to get the 2 additional bucks for another purchase. But this is only available if you can clear the previous debt condition, since you only get the 2 bucks when you gain the condition card.
EDIT: The official ruling is found in the Reference Guide on p. 4 under the "Conditions" header: "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." and "If a Condition’s effect does not list a limit, it may only be used once per instance of the triggered event."
It is probably discussed in the manual too, but I find it is easier to look things up in the Reference Guide, generally speaking.
The above should be understood in conjunction with the Bank Loan text on the board: "When performing an Acquire Assets action, you may gain a Debt Condition to immediately add 2 successes to your test result". Again, you can only gain a condition you don't have so this only triggers once, when you gain the debt.
As was already mentioned, the reference guide states:
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.
However, this rule does not prevent a player from making a choice to gain a condition, and then failing to actually gain it.
This issue is solved in the official errata:
In the Reference Guide, on page 4, the third bullet point under “Conditions” should read as follows: “An investigator cannot gain a copy of a Condition that he already has. An investigator cannot choose to gain a copy of a Condition that he already has.”
Whoa! You guys, you cannot take out a loan if you're in debt. That rule is there to mimic real life, where taking out a loan with bad credit is impossible. No the debt doesn't stack, but as long as you're in debt, you can't get the bank loan, because you can't gain the debt card, because you can't have more than one debt. From the reference guide under Bank Loans:
If an investigator already has a Debt Condition, he cannot acquire a bank loan.
Boom. Done. Pay me.