diff --git a/src/subgraph.js b/src/subgraph.js
index 53e402ab..a724f48c 100644
--- a/src/subgraph.js
+++ b/src/subgraph.js
@@ -391,6 +391,29 @@ More than one data source named '${name}', data source names must be unique.`,
     }, immutable.List())
   }
 
+  static validateUniqueDataSources(manifest) {
+    let data_source_identifiers = immutable.List()
+    return manifest.get('dataSources').reduce((errors, dataSource, dataSourceIndex) => {
+      let path = ['dataSources', dataSourceIndex]
+      let data_source_identifier = dataSource.get('source').merge(dataSource.get('mapping'))
+      if (data_source_identifiers.includes(data_source_identifier)) {
+        console.log("duplicate")
+        errors = errors.push(
+          immutable.fromJS({
+            path,
+            message: `\
+This data source, '${dataSource.get('name')}', has identical source and mapping to an already declared data source.
+They will both listen to the same triggers, process them with the same mappings, and generate the same entity updates.`,
+          }),
+        )
+      }
+
+      data_source_identifiers = data_source_identifiers.push(data_source_identifier)
+      return errors
+    }, immutable.List())
+  }
+
+
   static validateUniqueTemplateNames(manifest) {
     let names = []
     return manifest
@@ -460,7 +483,7 @@ More than one template named '${name}', template names must be unique.`,
       : immutable.List.of(
           ...Subgraph.validateRepository(manifest, { resolveFile }),
           ...Subgraph.validateDescription(manifest, { resolveFile }),
-          ...Subgraph.validateEthereumContractHandlers(manifest),
+          ...Subgraph.validateUniqueDataSource(manifest),
         )
 
     return {
diff --git a/tests/cli/validation.test.js b/tests/cli/validation.test.js
index f1947cd9..57246f8b 100644
--- a/tests/cli/validation.test.js
+++ b/tests/cli/validation.test.js
@@ -162,6 +162,15 @@ describe('Validation', () => {
     },
   )
 
+  cliTest(
+    'Duplicate data source functionality',
+    ['codegen', '--skip-migrations'],
+    'validation/duplicate-data-source-functionality',
+    {
+      exitCode: 0,
+    },
+  )
+
   cliTest(
     'Duplicate template name',
     ['codegen', '--skip-migrations'],
diff --git a/tests/cli/validation/duplicate-data-source-functionality.stderr b/tests/cli/validation/duplicate-data-source-functionality.stderr
new file mode 100644
index 00000000..9801c0b7
--- /dev/null
+++ b/tests/cli/validation/duplicate-data-source-functionality.stderr
@@ -0,0 +1,36 @@
+- Load subgraph from subgraph.yaml
+⚠ Warnings while loading subgraph from subgraph.yaml: Warnings in subgraph.yaml:
+
+    Path: dataSources > 1
+    This data source, 'ExampleSubgraph1', has identical source and mapping to an already declared data source.
+    They will both listen to the same triggers, process them with the same mappings, and generate the same entity updates.
+
+✔ Load subgraph from subgraph.yaml
+- Load contract ABIs
+  Load contract ABI from Abi.json
+- Load contract ABIs
+  Load contract ABI from Abi.json
+- Load contract ABIs
+✔ Load contract ABIs
+- Generate types for contract ABIs
+  Generate types for contract ABI: ExampleContract (Abi.json)
+- Generate types for contract ABIs
+  Write types to generated/ExampleSubgraph/ExampleContract.ts
+- Generate types for contract ABIs
+  Generate types for contract ABI: ExampleContract (Abi.json)
+- Generate types for contract ABIs
+  Write types to generated/ExampleSubgraph1/ExampleContract.ts
+- Generate types for contract ABIs
+✔ Generate types for contract ABIs
+- Generate types for data source templates
+✔ Generate types for data source templates
+- Load data source template ABIs
+✔ Load data source template ABIs
+- Generate types for data source template ABIs
+✔ Generate types for data source template ABIs
+- Load GraphQL schema from schema.graphql
+✔ Load GraphQL schema from schema.graphql
+- Generate types for GraphQL schema
+  Write types to generated/schema.ts
+- Generate types for GraphQL schema
+✔ Generate types for GraphQL schema
diff --git a/tests/cli/validation/duplicate-data-source-functionality/Abi.json b/tests/cli/validation/duplicate-data-source-functionality/Abi.json
new file mode 100644
index 00000000..4d05f583
--- /dev/null
+++ b/tests/cli/validation/duplicate-data-source-functionality/Abi.json
@@ -0,0 +1,7 @@
+[
+  {
+    "type": "event",
+    "name": "ExampleEvent",
+    "inputs": [{ "type": "string" }]
+  }
+]
diff --git a/tests/cli/validation/duplicate-data-source-functionality/mapping.ts b/tests/cli/validation/duplicate-data-source-functionality/mapping.ts
new file mode 100644
index 00000000..e69de29b
diff --git a/tests/cli/validation/duplicate-data-source-functionality/schema.graphql b/tests/cli/validation/duplicate-data-source-functionality/schema.graphql
new file mode 100644
index 00000000..218c5b9c
--- /dev/null
+++ b/tests/cli/validation/duplicate-data-source-functionality/schema.graphql
@@ -0,0 +1,4 @@
+type MyEntity @entity {
+  id: ID!
+  x: BigDecimal!
+}
diff --git a/tests/cli/validation/duplicate-data-source-functionality/subgraph.yaml b/tests/cli/validation/duplicate-data-source-functionality/subgraph.yaml
new file mode 100644
index 00000000..5ebf6dd0
--- /dev/null
+++ b/tests/cli/validation/duplicate-data-source-functionality/subgraph.yaml
@@ -0,0 +1,40 @@
+specVersion: 0.0.1
+repository: https://github.com/graphprotocol/test-subgraph
+description: Test subgraph
+schema:
+  file: ./schema.graphql
+dataSources:
+  - kind: ethereum/contract
+    name: ExampleSubgraph
+    source:
+      abi: ExampleContract
+    mapping:
+      kind: ethereum/events
+      apiVersion: 0.0.1
+      language: wasm/assemblyscript
+      file: ./mapping.ts
+      entities:
+        - ExampleEntity
+      abis:
+        - name: ExampleContract
+          file: ./Abi.json
+      eventHandlers:
+        - event: ExampleEvent(string)
+          handler: handleExampleEvent
+  - kind: ethereum/contract
+    name: ExampleSubgraph1
+    source:
+      abi: ExampleContract
+    mapping:
+      kind: ethereum/events
+      apiVersion: 0.0.1
+      language: wasm/assemblyscript
+      file: ./mapping.ts
+      entities:
+        - ExampleEntity
+      abis:
+        - name: ExampleContract
+          file: ./Abi.json
+      eventHandlers:
+        - event: ExampleEvent(string)
+          handler: handleExampleEvent