Relax registration checks for same servers somewhat.

If a server with the same ServerInfo is attempting to be re-registered,
we will return the existing registered server, which is less surprising
behavior than before.

proxy/src/main/java/com/velocitypowered/proxy/server/ServerMap.java

@@ -34,8 +34,15 @@ public class ServerMap {
         Preconditions.checkNotNull(serverInfo, "serverInfo");
         String lowerName = serverInfo.getName().toLowerCase(Locale.US);
         VelocityRegisteredServer rs = new VelocityRegisteredServer(server, serverInfo);
-        Preconditions.checkArgument(servers.putIfAbsent(lowerName, rs) == null, "Server with name %s already registered", serverInfo.getName());
+
+        RegisteredServer existing = servers.putIfAbsent(lowerName, rs);
+        if (existing != null && !existing.getServerInfo().equals(serverInfo)) {
+            throw new IllegalArgumentException("Server with name " + serverInfo.getName() + " already registered");
+        } else if (existing == null) {
             return rs;
+        } else {
+            return existing;
+        }
     }
 
     public void unregister(ServerInfo serverInfo) {

proxy/src/test/java/com/velocitypowered/proxy/util/ServerMapTest.java

@@ -35,4 +35,12 @@ class ServerMapTest {
         ServerInfo willReject = new ServerInfo("TESTSERVER", TEST_ADDRESS);
         assertThrows(IllegalArgumentException.class, () -> map.register(willReject));
     }
+
+    @Test
+    void allowsSameServerLaxRegistrationCheck() {
+        ServerMap map = new ServerMap(null);
+        ServerInfo info = new ServerInfo("TestServer", TEST_ADDRESS);
+        RegisteredServer connection = map.register(info);
+        assertEquals(connection, map.register(info));
+    }
 }