Fix Duplicate UTM Params: Remove Redundant Parameters in 3 Steps

function removeDuplicateUtm(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) {
            // Update to last value if keepFirst=false
            params.set(key, value);
        }
    }
 
    urlObj.search = params.toString();
    return urlObj.toString();
}
 
// Usage
const broken = 'https://site.com?utm_source=facebook&utm_campaign=spring&utm_source=instagram';
 
// Keep first occurrence
console.log(removeDuplicateUtm(broken, true));
// Output: https://site.com?utm_source=facebook&utm_campaign=spring
 
// Keep last occurrence
console.log(removeDuplicateUtm(broken, false));
// Output: https://site.com?utm_source=instagram&utm_campaign=spring

function batchFixDuplicates(urls) {
    return urls.map(url => {
        try {
            return removeDuplicateUtm(url, true);
        } catch (error) {
            console.error(`Failed to process: ${"{"}{"{"}url{"}"}{"}"}}`, error);
            return url; // Return original if fix fails
        }
    });
}
 
// Usage
const brokenUrls = [
    'site.com?utm_source=fb&utm_source=ig',
    'site.com?s=email&utm_campaign=a&utm_campaign=b',
    'site.com?utm_medium=cpc&other=x&utm_medium=paid'
];
 
const fixed = batchFixDuplicates(brokenUrls);
console.log(fixed);
// All duplicates removed, first occurrence kept

// Paste this in browser console when on the broken URL
(function() {
    const url = new URL(window.location.href);
    const params = new URLSearchParams();
    const seen = new Set();
 
    for (const [key, value] of url.searchParams.entries()) {
        if (!seen.has(key)) {
            params.append(key, value);
            seen.add(key);
        }
    }
 
    url.search = params.toString();
    window.location.href = url.toString();
})();
 
// Automatically redirects to cleaned URL

// Verify no duplicates remain
function verifyNoDuplicates(url) {
    const params = new URL(url).searchParams;
    const counts = {};
 
    for (const [key] of params.entries()) {
        counts[key] = (counts[key] || 0) + 1;
    }
 
    const duplicates = Object.entries(counts)
        .filter(([, count]) => count > 1)
        .map(([key]) => key);
 
    if (duplicates.length > 0) {
        console.error('❌ Duplicates still exist:', duplicates);
        return false;
    }
 
    console.log('✅ No duplicates found');
    return true;
}
 
// Usage
const url = 'https://site.com?utm_source=facebook&utm_campaign=spring';
verifyNoDuplicates(url);
// ✅ No duplicates found

// ✅ CORRECT: .set() replaces instead of duplicating
const url = new URL('https://site.com');
url.searchParams.set('utm_source', 'facebook');
url.searchParams.set('utm_medium', 'cpc');
 
// If you call .set() again with same key, it replaces:
url.searchParams.set('utm_source', 'instagram'); // Replaces 'facebook'
 
console.log(url.toString());
// https://site.com?utm_source=instagram&utm_medium=cpc

// ✅ CORRECT: Check existence before appending
function safeAddUtm(url, key, value) {
    const urlObj = new URL(url);
 
    if (urlObj.searchParams.has(key)) {
        console.warn(`${"{"}{"{"}key{"}"}{"}"}} already exists, replacing...`);
        urlObj.searchParams.set(key, value);
    } else {
        urlObj.searchParams.append(key, value);
    }
 
    return urlObj.toString();
}

// Add to pre-launch checklist
function preLaunchCheck(url) {
    const checks = [];
 
    // Check for duplicates
    const params = new URL(url).searchParams;
    const utmParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_content', 'utm_term'];
 
    utmParams.forEach(param => {
        const count = params.getAll(param).length;
        if (count > 1) {
            checks.push(`❌ Duplicate ${"{"}{"{"}param{"}"}{"}"}} (${"{"}{"{"}count{"}"}{"}"}} occurrences)`);
        } else if (count === 1) {
            checks.push(`✅ ${"{"}{"{"}param{"}"}{"}"}} OK`);
        }
    });
 
    return checks;
}

<a href="https://shop.com?utm_source=email&utm_campaign={{campaign.name}}?utm_source=newsletter&utm_medium=email">

<!-- Cleaned template -->
<a href="https://shop.com?utm_source=newsletter&utm_medium=email&utm_campaign={{campaign.name}}">

url.searchParams.set('utm_source', 'facebook'); // Never creates duplicates