Double Question Mark URL Error: Why GA4 Sees Corrupted Data

You checked your campaign URL. You saw TWO question marks:

shop.com?page=products?utm_source=facebook&utm_medium=cpc
           ↑            ↑
        First ?     Second ? (ERROR!)

GA4 shows corrupted campaign data. Some parameters tracked, others missing. Your attribution is a mess.

Two question marks breaks URL parsing. Here's why and how to fix it.

The Problem: Only One ? Allowed

URLs can have only ONE question mark starting the query string:

✅ CORRECT:
shop.com?param1=value1&param2=value2&param3=value3
         ↑                   ↑              ↑
      First ?             & separator    & separator

❌ WRONG:
shop.com?param1=value1?param2=value2&param3=value3
         ↑            ↑
      First ?    Second ? (breaks parsing)

Why This Happens

How Browsers Parse URLs

URL: shop.com?page=products&utm_source=facebook
              ↑
     Question mark starts query string

Query string: page=products&utm_source=facebook
Parameters:
  - page = products
  - utm_source = facebook

With Double Question Marks

URL: shop.com?page=products?utm_source=facebook
              ↑             ↑
           First ?     Treated as part of value!

Query string: page=products?utm_source=facebook
Parameters:
  - page = products?utm_source=facebook
  - utm_source = MISSING (not parsed)

The browser treats the second ? as part of the first parameter's value.

How This Mistake Happens

Scenario 1: Concatenating URLs with Existing Parameters

// Base URL already has parameters
const baseUrl = 'shop.com?page=products';
 
// ❌ WRONG - Adds another ?
const campaignUrl = baseUrl + '?utm_source=facebook';
// Result: shop.com?page=products?utm_source=facebook
 
// ✅ CORRECT - Check for existing ?
const campaignUrl = baseUrl + '&utm_source=facebook';
// Result: shop.com?page=products&utm_source=facebook

Scenario 2: URL Builder Logic Error

function buildCampaignUrl(base, utms) {
    // ❌ WRONG - Doesn't check if base has parameters
    return base + '?' + utms;
}
 
// Input: buildCampaignUrl('site.com?sort=price', 'utm_source=email')
// Output: site.com?sort=price?utm_source=email

Scenario 3: Copy-Paste Mistakes

You have: site.com?category=shoes
You want to add: ?utm_source=facebook&utm_medium=cpc

❌ Result: site.com?category=shoes?utm_source=facebook&utm_medium=cpc
                                 ↑
                          Double question mark

The Fix

Step 1: Find Double Question Marks

Search your campaign URLs for ?? or ?.*? patterns.

Step 2: Replace Second ? with &

❌ BEFORE:
shop.com?page=products?utm_source=facebook&utm_medium=cpc

✅ AFTER:
shop.com?page=products&utm_source=facebook&utm_medium=cpc
                      ↑
               Changed ? to &

Step 3: Use Smart Concatenation

function smartConcat(baseUrl, params) {
    // Check if baseUrl already has parameters
    const separator = baseUrl.includes('?') ? '&' : '?';
    return baseUrl + separator + params;
}
 
// Usage
smartConcat('site.com', 'utm_source=facebook');
// Output: site.com?utm_source=facebook
 
smartConcat('site.com?page=1', 'utm_source=facebook');
// Output: site.com?page=1&utm_source=facebook

Real Example: Email Platform Concatenation

Platform: Mailchimp campaign Problem: Email template with merge tag logic error

Template code:

<a href="{`{"{"}{"{"}website{"}"}{"}"}}`}?{"{"}{"{"}utm_parameters{"}"}{"}"}}">Click here</a>

Variables:

website = https://shop.com?category=products
utm_parameters = utm_source=mailchimp&utm_medium=email

Result:

https://shop.com?category=products?utm_source=mailchimp&utm_medium=email
                                  ↑
                          Double question mark

GA4 received:

page: category=products?utm_source=mailchimp&utm_medium=email
utm_source: (missing)
utm_medium: (missing)

Fix:

<a href="{`{"{"}{"{"}website{"}"}{"}"}}`}&{"{"}{"{"}utm_parameters{"}"}{"}"}}">Click here</a>
(Changed ? to & in template)

Or smarter template logic:

<a href="{`{"{"}{"{"}website{"}"}{"}"}}`}{{#if website contains '?'}}&{"{"}{"{"}else{"}"}{"}"}}?{{/if}}{"{"}{"{"}utm_parameters{"}"}{"}"}}">Click here</a>

FAQ

Can I have multiple question marks in a URL?

No. RFC 3986 defines only ONE question mark to start the query string. Additional parameters use &.

What if my parameter value contains a question mark?

URL-encode it: ?message=hello%3Fworld (%3F is encoded ?).

How do browsers handle multiple question marks?

They treat the second ? as part of the first parameter's value, not as a separator.

Will this break my links?

Yes. Parameter parsing breaks, causing tracking failures and potentially broken page functionality.

How do I prevent this in URL builders?

Add logic to check if the base URL already contains ? before adding parameters.

Conclusion

Double question marks break URL parameter parsing. Use only ONE ? to start the query string, then & for additional parameters.

❌ WRONG: site.com?param1=x?param2=y
✅ RIGHT: site.com?param1=x&param2=y

Fix in 30 seconds: Replace the second ? with &.

Technical Reference: Multiple Question Marks Validation Rule