diff --git a/server/src/main/java/com/cloud/resource/ResourceManagerImpl.java b/server/src/main/java/com/cloud/resource/ResourceManagerImpl.java
index cc789bf56508..3900fbab792e 100755
--- a/server/src/main/java/com/cloud/resource/ResourceManagerImpl.java
+++ b/server/src/main/java/com/cloud/resource/ResourceManagerImpl.java
@@ -798,6 +798,8 @@ private List<HostVO> discoverHostsFull(final Long dcId, final Long podId, Long c
             throw new InvalidParameterValueException(url + " is not a valid uri");
         }
 
+        checkForDuplicateHost(url);
+
         final List<HostVO> hosts = new ArrayList<>();
         logger.info("Trying to add a new host at {} in data center {}", url, zone);
         boolean isHypervisorTypeSupported = false;
@@ -2585,6 +2587,24 @@ private Host createHostAndAgent(final ServerResource resource, final Map<String,
         return host;
     }
 
+    void checkForDuplicateHost(final String url) {
+        String hostIpOrName = null;
+        try {
+            hostIpOrName = new URI(UriUtils.encodeURIComponent(url)).getHost();
+        } catch (final URISyntaxException ignore) {
+            // unparseable URL - discoverer will reject it shortly anyway
+        }
+        if (StringUtils.isBlank(hostIpOrName)) {
+            return;
+        }
+        final HostVO existingByIp = _hostDao.findByIp(hostIpOrName);
+        if (existingByIp != null) {
+            throw new InvalidParameterValueException(String.format(
+                    "A host with IP address / hostname '%s' already exists (id: %s). Remove it before adding again.",
+                    hostIpOrName, existingByIp.getUuid()));
+        }
+    }
+
     private Host createHostAndAgentDeferred(final ServerResource resource, final Map<String, String> details, final boolean old, final List<String> hostTags, final boolean forRebalance) {
         HostVO host = null;
         StartupCommand[] cmds = null;
diff --git a/server/src/test/java/com/cloud/resource/ResourceManagerImplTest.java b/server/src/test/java/com/cloud/resource/ResourceManagerImplTest.java
index 91e4bf7a47b8..28b156e03ea5 100644
--- a/server/src/test/java/com/cloud/resource/ResourceManagerImplTest.java
+++ b/server/src/test/java/com/cloud/resource/ResourceManagerImplTest.java
@@ -17,6 +17,46 @@
 
 package com.cloud.resource;
 
+import static com.cloud.resource.ResourceState.Event.ErrorsCorrected;
+import static com.cloud.resource.ResourceState.Event.InternalEnterMaintenance;
+import static com.cloud.resource.ResourceState.Event.UnableToMaintain;
+import static com.cloud.resource.ResourceState.Event.UnableToMigrate;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyBoolean;
+import static org.mockito.ArgumentMatchers.anyLong;
+import static org.mockito.ArgumentMatchers.anyString;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import java.lang.reflect.Field;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.UUID;
+
+import org.apache.cloudstack.api.command.admin.host.CancelHostAsDegradedCmd;
+import org.apache.cloudstack.api.command.admin.host.DeclareHostAsDegradedCmd;
+import org.apache.cloudstack.framework.config.ConfigKey;
+import org.apache.cloudstack.framework.config.dao.ConfigurationDao;
+import org.junit.After;
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.BDDMockito;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.MockedConstruction;
+import org.mockito.MockedStatic;
+import org.mockito.Mockito;
+import org.mockito.MockitoAnnotations;
+import org.mockito.Spy;
+import org.mockito.junit.MockitoJUnitRunner;
+
 import com.cloud.agent.AgentManager;
 import com.cloud.agent.api.GetVncPortAnswer;
 import com.cloud.agent.api.GetVncPortCommand;
@@ -43,45 +83,6 @@
 import com.cloud.vm.dao.UserVmDetailsDao;
 import com.cloud.vm.dao.VMInstanceDao;
 import com.trilead.ssh2.Connection;
-import org.apache.cloudstack.api.command.admin.host.CancelHostAsDegradedCmd;
-import org.apache.cloudstack.api.command.admin.host.DeclareHostAsDegradedCmd;
-import org.apache.cloudstack.framework.config.ConfigKey;
-import org.apache.cloudstack.framework.config.dao.ConfigurationDao;
-import org.junit.After;
-import org.junit.Assert;
-import org.junit.Before;
-import org.junit.Test;
-import org.junit.runner.RunWith;
-import org.mockito.BDDMockito;
-import org.mockito.InjectMocks;
-import org.mockito.Mock;
-import org.mockito.MockedConstruction;
-import org.mockito.MockedStatic;
-import org.mockito.Mockito;
-import org.mockito.MockitoAnnotations;
-import org.mockito.Spy;
-import org.mockito.junit.MockitoJUnitRunner;
-
-import java.lang.reflect.Field;
-import java.util.ArrayList;
-import java.util.Arrays;
-import java.util.Collections;
-import java.util.List;
-import java.util.UUID;
-
-import static com.cloud.resource.ResourceState.Event.ErrorsCorrected;
-import static com.cloud.resource.ResourceState.Event.InternalEnterMaintenance;
-import static com.cloud.resource.ResourceState.Event.UnableToMaintain;
-import static com.cloud.resource.ResourceState.Event.UnableToMigrate;
-import static org.mockito.ArgumentMatchers.any;
-import static org.mockito.ArgumentMatchers.anyBoolean;
-import static org.mockito.ArgumentMatchers.anyLong;
-import static org.mockito.ArgumentMatchers.anyString;
-import static org.mockito.ArgumentMatchers.eq;
-import static org.mockito.Mockito.never;
-import static org.mockito.Mockito.times;
-import static org.mockito.Mockito.verify;
-import static org.mockito.Mockito.when;
 
 @RunWith(MockitoJUnitRunner.class)
 public class ResourceManagerImplTest {
@@ -218,6 +219,19 @@ public void tearDown() throws Exception {
         closeable.close();
     }
 
+    @Test(expected = InvalidParameterValueException.class)
+    public void testCheckForDuplicateHostThrowsWhenIpAlreadyExists() {
+        when(hostDao.findByIp("10.0.0.10")).thenReturn(host);
+        resourceManager.checkForDuplicateHost("http://10.0.0.10");
+    }
+
+    @Test
+    public void testCheckForDuplicateHostAllowsUniqueHost() {
+        when(hostDao.findByIp("10.0.0.30")).thenReturn(null);
+        resourceManager.checkForDuplicateHost("http://10.0.0.30");
+        verify(hostDao, times(1)).findByIp("10.0.0.30");
+    }
+
     @Test
     public void testCheckAndMaintainEnterMaintenanceModeNoVms() throws NoTransitionException {
         // Test entering into maintenance with no VMs running on host.