Duplicate UTM Parameters Detected: Which Value Does GA4 Use?

// ❌ WRONG: Adds parameters without checking for duplicates
const baseUrl = 'site.com?utm_source=google&utm_medium=cpc';
const additionalParams = '&utm_source=facebook';
const finalUrl = baseUrl + additionalParams;
 
// Result: ?utm_source=google&utm_medium=cpc&utm_source=facebook
//          ↑                                  ↑
//          Duplicate utm_source

<!-- ❌ WRONG: Template adds UTMs, but URL already has them -->
<a href="{`{"{"}{"{"}landing_page{"}"}{"}"}}`}?utm_source=email&utm_campaign=newsletter">
    ↑
    If landing_page = "site.com?utm_source=social", you get duplicates

function findDuplicateParams(url) {
    const params = new URL(url).searchParams;
    const seen = new Set();
    const duplicates = [];
 
    for (const [key] of params.entries()) {
        if (seen.has(key)) {
            duplicates.push(key);
        }
        seen.add(key);
    }
 
    return duplicates;
}
 
// Usage
const url = 'site.com?utm_source=fb&utm_medium=cpc&utm_source=ig';
console.log(findDuplicateParams(url));
// Output: ['utm_source']

function removeDuplicateParams(url, keepFirst = true) {
    const urlObj = new URL(url);
    const params = new URLSearchParams();
    const seen = new Set();
 
    for (const [key, value] of urlObj.searchParams.entries()) {
        if (!seen.has(key)) {
            params.append(key, value);
            seen.add(key);
        } else if (!keepFirst) {
            // If keepFirst=false, update to last value
            params.set(key, value);
        }
    }
 
    urlObj.search = params.toString();
    return urlObj.toString();
}
 
// Usage - keep first occurrence
const url = 'https://site.com?utm_source=facebook&utm_medium=cpc&utm_source=instagram';
console.log(removeDuplicateParams(url, true));
// Output: https://site.com?utm_source=facebook&utm_medium=cpc
 
// Or keep last occurrence
console.log(removeDuplicateParams(url, false));
// Output: https://site.com?utm_source=instagram&utm_medium=cpc

// ✅ CORRECT: Check if parameter exists before adding
function addUtmParam(url, key, value) {
    const urlObj = new URL(url);
 
    // Remove existing parameter if present
    if (urlObj.searchParams.has(key)) {
        urlObj.searchParams.delete(key);
    }
 
    // Add new value
    urlObj.searchParams.append(key, value);
 
    return urlObj.toString();
}
 
// Usage
let url = 'https://site.com?utm_source=google';
url = addUtmParam(url, 'utm_source', 'facebook');
// Result: https://site.com?utm_source=facebook (replaced, not duplicated)

// ✅ CORRECT: .set() replaces instead of duplicating
const url = new URL('https://site.com?utm_source=google');
url.searchParams.set('utm_source', 'facebook'); // Replaces google with facebook
url.searchParams.set('utm_medium', 'cpc');       // Adds new parameter
 
console.log(url.toString());
// https://site.com?utm_source=facebook&utm_medium=cpc

// Add to URL builder validation
function validateUtmUrl(url) {
    const params = new URL(url).searchParams;
    const utmParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_content', 'utm_term'];
 
    const duplicates = utmParams.filter(param => {
        return params.getAll(param).length > 1;
    });
 
    if (duplicates.length > 0) {
        throw new Error(`Duplicate parameters detected: ${duplicates.join(', ')}`);
    }
 
    return true;
}

// Express.js middleware to clean duplicate UTM parameters
function cleanUtmParams(req, res, next) {
    const utmParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_content', 'utm_term'];
 
    utmParams.forEach(param => {
        if (Array.isArray(req.query[param])) {
            // If duplicate, keep first value
            req.query[param] = req.query[param][0];
        }
    });
 
    next();
}
 
app.use(cleanUtmParams);

// Add to pre-launch validation
function auditUrl(url) {
    const issues = [];
    const params = new URL(url).searchParams;
    const paramCounts = {};
 
    // Count occurrences
    for (const [key] of params.entries()) {
        paramCounts[key] = (paramCounts[key] || 0) + 1;
    }
 
    // Report duplicates
    Object.entries(paramCounts).forEach(([key, count]) => {
        if (count > 1) {
            issues.push({
                parameter: key,
                occurrences: count,
                severity: 'critical'
            });
        }
    });
 
    return issues;
}