Short answer. Your right to recover from the borrower is seriously weakened. Under Article 2068 of the Civil Code, when a guarantor pays without notifying the debtor and the debtor then pays the creditor a second time, the debtor can raise against the guarantor every defense he could have raised against the creditor.
What the law says
If the guarantor should pay without notifying the debtor, the latter may enforce against him all the defenses which he could have set up against the creditor at the time the payment was made.
Civil Code, Article 2068 — Payment Without Notice. Read the full provision →
What Article 2068 actually says
The Civil Code imposes a notification duty on guarantors who pay a debt. Article 2068 states: "If the guarantor should pay without notifying the debtor, the latter may enforce against him all the defenses which he could have set up against the creditor at the time the payment was made." In plain terms, the guarantor who skips that notice steps into a disadvantaged position — the debtor can use any argument he would have had against the lender to resist the guarantor's claim for reimbursement.
Why the notice requirement exists
The debtor has no way of knowing the guarantor has already settled the account unless told. If the debtor is kept in the dark, he may — entirely in good faith — pay the creditor himself, as happened in your situation. Without the notification rule, the debtor would be trapped: he paid the creditor honestly, yet the guarantor could still chase him for the same amount. Article 2068 allocates the loss to the party who created the problem — the guarantor who failed to give notice.
What defenses the debtor can raise
The defenses available to the debtor are those he could have set up against the creditor at the time the guarantor paid. Examples rooted in the Civil Code include prescription of the debt, payment already made, compensation (set-off) against a counterclaim the debtor held against the creditor, or the fact that the debt was already extinguished. If the debt had been fully paid at the moment the guarantor stepped in, the guarantor's payment was unnecessary — and the debtor can say exactly that.
Your practical position right now
If the debtor genuinely paid a second time after your unnotified payment, he can argue the debt was already gone and resist your reimbursement claim. You are not necessarily left with nothing — you likely have a claim against the creditor who accepted double payment, on the basis of unjust enrichment. But pursuing the debtor directly for reimbursement will face the statutory defenses Article 2068 arms him with. Going forward, always notify the debtor in writing before or immediately upon making payment as guarantor, so the reimbursement right is clear and the debtor cannot claim ignorance.