diff --git a/packages/js/client/src/Web3ApiClient.ts b/packages/js/client/src/Web3ApiClient.ts
index 6e4cf11d6c..22dab4569c 100644
--- a/packages/js/client/src/Web3ApiClient.ts
+++ b/packages/js/client/src/Web3ApiClient.ts
@@ -100,6 +100,8 @@ export class Web3ApiClient implements Client {
 
       this._validateConfig();
 
+      this._sanitizeConfig();
+
       Tracer.setAttribute("config", this._config);
     } catch (error) {
       Tracer.recordException(error);
@@ -362,9 +364,7 @@ export class Web3ApiClient implements Client {
     if (freq && (freq.ms || freq.sec || freq.min || freq.hours)) {
       frequency =
         (freq.ms ?? 0) +
-        ((freq.hours ?? 0) * 3600 +
-          (freq.min ?? 0) * 60 +
-          (freq.sec ?? 0)) *
+        ((freq.hours ?? 0) * 3600 + (freq.min ?? 0) * 60 + (freq.sec ?? 0)) *
           1000;
     } else {
       frequency = 60000;
@@ -525,6 +525,64 @@ export class Web3ApiClient implements Client {
     }
   }
 
+  @Tracer.traceMethod("Web3ApiClient: sanitizeConfig")
+  private _sanitizeConfig(): void {
+    this._sanitizeInterfacesAndImplementations();
+  }
+
+  // Make sure interface URIs are unique and that all of their implementation URIs are unique
+  // If not, then merge them
+  @Tracer.traceMethod("Web3ApiClient: sanitizeInterfacesAndImplementations")
+  private _sanitizeInterfacesAndImplementations(): void {
+    const interfaces = this._config.interfaces;
+    // Interface hash map used to keep track of interfaces with same URI
+    // A set is used to keep track of unique implementation URIs
+    const addedInterfacesHashMap = new Map<string, Set<string>>();
+
+    for (const interfaceImplementations of interfaces) {
+      const interfaceUri = interfaceImplementations.interface.uri;
+
+      if (!addedInterfacesHashMap.has(interfaceUri)) {
+        // If the interface is not added yet then just add it along with its implementations
+        addedInterfacesHashMap.set(
+          interfaceUri,
+          new Set(interfaceImplementations.implementations.map((x) => x.uri))
+        );
+      } else {
+        const existingInterfaceImplementations = addedInterfacesHashMap.get(
+          interfaceUri
+        ) as Set<string>;
+
+        // Get implementations to add to existing set of implementations
+        const newImplementationUris = interfaceImplementations.implementations.map(
+          (x) => x.uri
+        );
+
+        // Add new implementations to existing set
+        newImplementationUris.forEach(
+          existingInterfaceImplementations.add,
+          existingInterfaceImplementations
+        );
+      }
+    }
+
+    // Collection of unique interfaces with implementations merged
+    const sanitizedInterfaces: InterfaceImplementations<Uri>[] = [];
+
+    // Go through the unique hash map of interfaces and implementations and add them to the sanitized interfaces
+    for (const [
+      interfaceUri,
+      implementationSet,
+    ] of addedInterfacesHashMap.entries()) {
+      sanitizedInterfaces.push({
+        interface: new Uri(interfaceUri),
+        implementations: [...implementationSet].map((x) => new Uri(x)),
+      });
+    }
+
+    this._config.interfaces = sanitizedInterfaces;
+  }
+
   @Tracer.traceMethod("Web3ApiClient: validateConfig")
   private _validateConfig(): void {
     // Require plugins to use non-interface URIs
diff --git a/packages/js/client/src/__tests__/Web3ApiClient.spec.ts b/packages/js/client/src/__tests__/Web3ApiClient.spec.ts
index a6eb70c16f..a3dd0f4ccb 100644
--- a/packages/js/client/src/__tests__/Web3ApiClient.spec.ts
+++ b/packages/js/client/src/__tests__/Web3ApiClient.spec.ts
@@ -2810,7 +2810,10 @@ enum Logger_LogLevel @imported(
   it("e2e Interface invoke method", async () => {
     const interfaceUri = "w3://ens/interface.eth";
     // Build interface polywrapper
-    await runCLI({ args: ["build"], cwd: `${GetPathToTestApis()}/interface-invoke/test-interface`});
+    await runCLI({
+      args: ["build"],
+      cwd: `${GetPathToTestApis()}/interface-invoke/test-interface`,
+    });
 
     const implementationApi = await buildAndDeployApi(
       `${GetPathToTestApis()}/interface-invoke/test-implementation`,
@@ -2824,7 +2827,7 @@ enum Logger_LogLevel @imported(
         {
           interface: interfaceUri,
           implementations: [implementationUri],
-        }
+        },
       ],
     });
 
@@ -2864,7 +2867,7 @@ enum Logger_LogLevel @imported(
         mutationMethod(
           arg: 1
         )
-      }`
+      }`,
     });
 
     expect(mutation.errors).toBeFalsy();
@@ -2872,4 +2875,68 @@ enum Logger_LogLevel @imported(
     expect(mutation.data?.mutationMethod).toBe(1);
   });
 
+  it("merge user-defined interface implementations with each other", async () => {
+    const interfaceUri = "w3://ens/interface.eth";
+    const implementationUri1 = "w3://ens/implementation1.eth";
+    const implementationUri2 = "w3://ens/implementation2.eth";
+
+    const client = new Web3ApiClient({
+      interfaces: [
+        {
+          interface: interfaceUri,
+          implementations: [implementationUri1],
+        },
+        {
+          interface: interfaceUri,
+          implementations: [implementationUri2],
+        },
+      ],
+    });
+
+    const interfaces = client
+      .getInterfaces()
+      .filter((x) => x.interface.uri === interfaceUri);
+    expect(interfaces.length).toEqual(1);
+
+    const implementationUris = interfaces[0].implementations;
+
+    expect(implementationUris).toEqual([
+      new Uri(implementationUri1),
+      new Uri(implementationUri2),
+    ]);
+  });
+
+  it("merge user-defined interface implementations with defaults", async () => {
+    const interfaceUri = coreInterfaceUris.uriResolver.uri;
+    const implementationUri1 = "w3://ens/implementation1.eth";
+    const implementationUri2 = "w3://ens/implementation2.eth";
+
+    const client = new Web3ApiClient({
+      interfaces: [
+        {
+          interface: interfaceUri,
+          implementations: [implementationUri1],
+        },
+        {
+          interface: interfaceUri,
+          implementations: [implementationUri2],
+        },
+      ],
+    });
+
+    const interfaces = client
+      .getInterfaces()
+      .filter((x) => x.interface.uri === interfaceUri);
+    expect(interfaces.length).toEqual(1);
+
+    const implementationUris = interfaces[0].implementations;
+
+    expect(implementationUris).toEqual([
+      new Uri(implementationUri1),
+      new Uri(implementationUri2),
+      ...getDefaultClientConfig().interfaces.find(
+        (x) => x.interface.uri === interfaceUri
+      )!.implementations,
+    ]);
+  });
 });