Test for duplicate transport versions (elastic#130494)

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;
@@ -216,4 +217,16 @@ 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;
+        }
+    }
 }