diff --git a/packages/repeater/src/request-runner/Request.spec.ts b/packages/repeater/src/request-runner/Request.spec.ts
index 16613a24..61df7918 100644
--- a/packages/repeater/src/request-runner/Request.spec.ts
+++ b/packages/repeater/src/request-runner/Request.spec.ts
@@ -84,5 +84,18 @@ describe('Request', () => {
         'x-a2': 'a2'
       });
     });
+
+    it('should join headers if multiple values is present', () => {
+      const request = new Request({
+        url: 'http://foo.bar',
+        protocol: Protocol.HTTP
+      });
+
+      request.setHeaders({ host: ['example.com', 'example1.com'] });
+
+      expect(request.headers).toEqual({
+        host: 'example.com, example1.com'
+      });
+    });
   });
 });
diff --git a/packages/repeater/src/request-runner/Request.ts b/packages/repeater/src/request-runner/Request.ts
index c7af8694..176ba768 100644
--- a/packages/repeater/src/request-runner/Request.ts
+++ b/packages/repeater/src/request-runner/Request.ts
@@ -11,6 +11,22 @@ export interface RequestOptions {
 }
 
 export class Request {
+  public static readonly SINGLE_VALUE_HEADERS: ReadonlySet<string> =
+    new Set<string>([
+      'authorization',
+      'content-disposition',
+      'content-length',
+      'content-type',
+      'from',
+      'host',
+      'if-modified-since',
+      'if-unmodified-since',
+      'location',
+      'max-forwards',
+      'proxy-authorization',
+      'referer',
+      'user-agent'
+    ]);
   public readonly protocol: Protocol;
   public readonly url: string;
   public readonly body?: string;
@@ -46,16 +62,28 @@ export class Request {
     this.url = url;
     this.correlationIdRegex =
       this.normalizeCorrelationIdRegex(correlationIdRegex);
-    this._headers = headers;
+    this.setHeaders(headers);
     this.precheckBody(body);
     this.body = body;
   }
 
   public setHeaders(headers: Record<string, string | string[]>): void {
-    this._headers = {
+    const mergedHeaders = {
       ...this._headers,
       ...headers
     };
+
+    this._headers = Object.fromEntries(
+      Object.entries(mergedHeaders).map(
+        ([field, value]: [string, string | string[]]) => [
+          field,
+          Array.isArray(value) &&
+          Request.SINGLE_VALUE_HEADERS.has(field.toLowerCase())
+            ? value.join(', ')
+            : value
+        ]
+      )
+    );
   }
 
   private validateUrl(url: string): void {