diff --git a/sign.js b/sign.js
index 82bf526..da1377a 100644
--- a/sign.js
+++ b/sign.js
@@ -214,7 +214,7 @@ module.exports = function (payload, secretOrPrivateKey, options, callback) {
     }
   }
 
-  Object.keys(options_to_payload).forEach(function (key) {
+  for (const key of Object.keys(options_to_payload)) {
     const claim = options_to_payload[key];
     if (typeof options[key] !== 'undefined') {
       if (typeof payload[claim] !== 'undefined') {
@@ -222,7 +222,7 @@ module.exports = function (payload, secretOrPrivateKey, options, callback) {
       }
       payload[claim] = options[key];
     }
-  });
+  }
 
   const encoding = options.encoding || 'utf8';
 
diff --git a/test/issue_1000.tests.js b/test/issue_1000.tests.js
new file mode 100644
index 0000000..2b4630c
--- /dev/null
+++ b/test/issue_1000.tests.js
@@ -0,0 +1,112 @@
+'use strict';
+
+const jwt = require('../');
+const expect = require('chai').expect;
+
+describe('issue 1000 - callback executed twice on payload conflict', function() {
+  it('should call the callback only once when payload has conflicting "iss" property', function(done) {
+    let callCount = 0;
+
+    jwt.sign(
+      { iss: 'bar', iat: 1757476476 },
+      'secret',
+      { algorithm: 'HS256', issuer: 'foo' },
+      (err, token) => {
+        callCount++;
+
+        // The callback should only be called once
+        if (callCount > 1) {
+          done(new Error(`Callback was called ${callCount} times instead of once`));
+          return;
+        }
+
+        // Give time for potential second callback
+        setTimeout(() => {
+          expect(callCount).to.equal(1);
+          expect(err).to.be.instanceOf(Error);
+          expect(err.message).to.equal('Bad "options.issuer" option. The payload already has an "iss" property.');
+          expect(token).to.be.undefined;
+          done();
+        }, 50);
+      }
+    );
+  });
+
+  it('should call the callback only once when payload has conflicting "sub" property', function(done) {
+    let callCount = 0;
+
+    jwt.sign(
+      { sub: 'bar' },
+      'secret',
+      { algorithm: 'HS256', subject: 'foo' },
+      (err, token) => {
+        callCount++;
+
+        if (callCount > 1) {
+          done(new Error(`Callback was called ${callCount} times instead of once`));
+          return;
+        }
+
+        setTimeout(() => {
+          expect(callCount).to.equal(1);
+          expect(err).to.be.instanceOf(Error);
+          expect(err.message).to.equal('Bad "options.subject" option. The payload already has an "sub" property.');
+          expect(token).to.be.undefined;
+          done();
+        }, 50);
+      }
+    );
+  });
+
+  it('should call the callback only once when payload has conflicting "aud" property', function(done) {
+    let callCount = 0;
+
+    jwt.sign(
+      { aud: 'bar' },
+      'secret',
+      { algorithm: 'HS256', audience: 'foo' },
+      (err, token) => {
+        callCount++;
+
+        if (callCount > 1) {
+          done(new Error(`Callback was called ${callCount} times instead of once`));
+          return;
+        }
+
+        setTimeout(() => {
+          expect(callCount).to.equal(1);
+          expect(err).to.be.instanceOf(Error);
+          expect(err.message).to.equal('Bad "options.audience" option. The payload already has an "aud" property.');
+          expect(token).to.be.undefined;
+          done();
+        }, 50);
+      }
+    );
+  });
+
+  it('should call the callback only once when payload has conflicting "jti" property', function(done) {
+    let callCount = 0;
+
+    jwt.sign(
+      { jti: 'bar' },
+      'secret',
+      { algorithm: 'HS256', jwtid: 'foo' },
+      (err, token) => {
+        callCount++;
+
+        if (callCount > 1) {
+          done(new Error(`Callback was called ${callCount} times instead of once`));
+          return;
+        }
+
+        setTimeout(() => {
+          expect(callCount).to.equal(1);
+          expect(err).to.be.instanceOf(Error);
+          expect(err.message).to.equal('Bad "options.jwtid" option. The payload already has an "jti" property.');
+          expect(token).to.be.undefined;
+          done();
+        }, 50);
+      }
+    );
+  });
+});