diff --git a/lemur/certificates/schemas.py b/lemur/certificates/schemas.py
index 8f15542d..bc7ff1b0 100644
--- a/lemur/certificates/schemas.py
+++ b/lemur/certificates/schemas.py
@@ -146,7 +146,9 @@ class CertificateInputSchema(CertificateCreationSchema):
                 data["extensions"]["subAltNames"] = {"names": []}
             elif not data["extensions"]["subAltNames"].get("names"):
                 data["extensions"]["subAltNames"]["names"] = []
-            data["extensions"]["subAltNames"]["names"] += csr_sans
+
+            if not data["extensions"]["subAltNames"]["names"]:
+                data["extensions"]["subAltNames"]["names"] += csr_sans
         return missing.convert_validity_years(data)