check-doc-syntax: Find duplicate "Since:" tags

It makes no sense to say more than one time when some symbol was introduced.

Signed-off-by: Uli Schlachter <psychon@znc.in>

src/check-doc-syntax.awk

@@ -69,6 +69,9 @@ in_doc {
 }
 
 /^ \* Since: ([0-9]*.[0-9]*|TBD)$/ {
+    if (doc_has_since != 0) {
+	log_error("Duplicate 'Since' field")
+    }
     doc_has_since = doc_line
 }
 
@@ -102,4 +105,4 @@ in_doc {
 END {
     if (!name_found)
 	log_warning("not found")
-}
+}