Test for duplicate transport versions

We used to have an assertion during transport version loading that
duplicate ids were not found, but it appears to have been lost in
refactorings. This commit adds a test to ensure duplicate ids do not
occur.

relates elastic#130486

Author: rjernst

server/src/test/java/org/elasticsearch/TransportVersionTests.java

@@ -13,6 +13,7 @@
 import org.elasticsearch.test.TransportVersionUtils;
 
 import java.lang.reflect.Modifier;
+import java.util.List;
 import java.util.Set;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -207,4 +208,16 @@ public void testToString() {
         assertEquals("2000099", TransportVersion.fromId(2_00_00_99).toString());
         assertEquals("5000099", TransportVersion.fromId(5_00_00_99).toString());
     }
+
+    public void testDuplicateConstants() {
+        List<TransportVersion> tvs = TransportVersion.getAllVersions();
+        TransportVersion previous = tvs.get(0);
+        for (int i = 1; i < tvs.size(); i++) {
+            TransportVersion next = tvs.get(i);
+            if (next.id() == previous.id()) {
+                throw new AssertionError("Duplicate transport version id: " + next.id());
+            }
+            previous = next;
+        }
+    }
 }