Short answer. When a will names an heir but two people share that name, Article 843 of the Civil Code requires the testator to add a distinguishing circumstance. If none was added, the institution may still be valid if the will as a whole leaves no real doubt about which person was meant.
What the law says
when there are two persons having the same names, he shall indicate some circumstance by which the instituted heir may be known
Civil Code, Article 843 — Naming the Heir. Read the full provision →
What the Civil Code requires when two people share a name
Article 843 says the testator must designate the heir by name and surname, and "when there are two persons having the same names, he shall indicate some circumstance by which the instituted heir may be known." A distinguishing circumstance can be a relationship to the testator ("my eldest son"), a place of residence, an occupation, or anything that singles out one person over another. The duty was on your father at the time he wrote the will.
The saving clause — doubt is the real test
The same article contains an important saving rule: even if the testator omitted the heir's name entirely, the institution is still valid if the will designates the person "in such manner that there can be no doubt as to who has been instituted." This means a will that names "Jose Santos" without more is not automatically void. The question is whether the rest of the will — its recitals, family descriptions, property descriptions, or the overall context — makes clear which Jose Santos was intended. If the answer is yes, the institution stands.
What happens when there is genuine doubt
If the will genuinely does not resolve which person was meant — if both Jose Santos claimants fit equally well — the institution becomes ambiguous and the estate cannot simply be divided between them as a workaround. The ambiguity must be resolved through the probate proceeding. The probate court examines all available evidence of the testator's intent: testimony about family relationships, prior statements, the context of property described in the will, and any other relevant circumstances. Neither claimant can simply take half without a ruling.
Practical steps if you are facing this situation
If probate has not yet been filed, gather every piece of evidence that points to your father's intent — letters, earlier draft documents, property descriptions, witness accounts. If probate is already underway, raise the ambiguity formally before the court rather than waiting for the other claimant to do so. The estate administrator or executor has an obligation to present the issue. An attorney familiar with estate proceedings can help you assess whether the will's overall content supplies the missing distinction, or whether the ambiguity is one the court must resolve on evidence.