different tags could be written to the same file

There was a missing synchronization in the code that maps
Strings to Integers.

pdb-api/src/main/java/org/lucares/pdb/api/UniqueStringIntegerPairs.java

@@ -121,8 +121,12 @@ public class UniqueStringIntegerPairs {
 	public Integer computeIfAbsent(final String first, final Function<String, Integer> mappingFunction) {
 
 		if (!stringToInt.containsKey(first)) {
-			final Integer second = mappingFunction.apply(first);
-			put(first, second);
+			synchronized (stringToInt) {
+				if (!stringToInt.containsKey(first)) {
+					final Integer second = mappingFunction.apply(first);
+					put(first, second);
+				}
+			}
 		}
 
 		return stringToInt.get(first);