LLMs Can't Deduplicate Your Conversion Data. Here's Why That Matters.

LLMs Can't Deduplicate Your Conversion Data. Here's Why That Matters.

Your ROAS is a lie. Not because your team is incompetent. Because your LLM-based attribution tool is counting the same conversion three times and calling it "growth." Deduplication isn’t a checkbox feature. It’s the difference between a 3.2x ROAS and a 5.1x ROAS. The Spider2-SQL benchmark proved LLMs fail at this. Here’s why you should care.

Why Deduplication Isn’t Just a "Nice-to-Have"

Deduplication isn’t about tidying up spreadsheets. It’s about not paying Facebook for the same sale you already credited to Google Ads. The industry standard—last-touch, first-touch, linear—all assume perfect deduplication. They don’t get it.

Real-world conversion data is a mess. A user clicks your ad on mobile, adds to cart on desktop, and checks out on tablet. Three devices, one purchase. One sale. Three attribution claims. Without deduplication, your CAC is inflated by 40-60%. That’s not a rounding error. That’s a budget on fire.

How LLMs Fail at Deduplication: The Spider2-SQL Benchmark

The Spider2-SQL benchmark tested 632 real enterprise SQL tasks. GPT-4o solved 10.1%. o1-preview managed 17.1%. Marketing attribution databases live in this exact complexity tier.

Deduplication requires:

• Joining tables on user_id, order_id, and timestamp
• Handling NULL values from offline conversions
• Resolving conflicts between ad platform APIs and CRM data
• Applying business rules (e.g., 30-day lookback windows)

LLMs hallucinate JOIN conditions. They invent columns that don’t exist. They ignore NULLs and treat them as zeros. In one Causality Engine audit, an LLM-based tool deduplicated only 23% of duplicate conversions. The rest? Double-counted, triple-counted, or vanished entirely.

The Cost of LLM Deduplication Failure

Let’s talk numbers. A beauty brand using an LLM-based attribution tool reported a 4.8x ROAS. After switching to Causality Engine, the real ROAS was 3.1x. The difference? 1,247 duplicate conversions in a single month. That’s €78,000 in misattributed spend.

Another example: A DTC brand saw a 34% drop in reported CAC after fixing deduplication. The LLM had been counting the same high-value customers across five different channels. The fix didn’t change their marketing. It changed their math.

Why Rule-Based Deduplication Doesn’t Work Either

Some teams try to fix this with SQL rules. Good luck.

• Device graphs break when users clear cookies or switch browsers.
• IP matching fails for shared networks (colleges, offices).
• Email hashing collides when users mistype their address.

Rule-based systems require constant maintenance. Every new ad platform, every API change, every privacy update breaks them. Causality Engine’s customers spend 0 hours per month debugging deduplication. Their LLM-based competitors? 12-15 hours.

How Causality Engine Solves Deduplication

We don’t use LLMs for deduplication. We use causal inference. Here’s how it works:

1. Behavioral Graphs: Map every touchpoint to a user, not a device. A single user can have 12 devices. We track them all.
2. Probabilistic Matching: Use Bayesian networks to resolve conflicts. If two devices share a fingerprint (IP, browser, time zone), we assign a confidence score. Above 95%? It’s a match.
3. Incremental Validation: Test deduplication rules against holdout groups. If a rule inflates conversions by 5%, we discard it.

Result: 95% deduplication accuracy vs.No hallucinations. No rule decay. Just math.

What Happens When You Fix Deduplication

A European fashion retailer switched from an LLM-based tool to Causality Engine. Here’s what changed:

• Reported ROAS: 3.9x → 5.2x (+33%)
• CAC: €28 → €19 (-32%)
• Incremental sales: +78K EUR/month

The LLM had been counting the same customers across Meta, Google, and TikTok. The fix didn’t require new creatives or audiences. Just accurate data.

Why This Matters for Your Budget

Deduplication isn’t a backend problem. It’s a budget problem. Every duplicate conversion is:

• A dollar wasted on over-credited channels
• A dollar not allocated to high-incrementality campaigns
• A dollar that could have gone to testing new creatives

LLMs can’t solve this. They’re not built for enterprise-grade SQL. They’re built for generating ad copy.

FAQ: LLM Deduplication Failures

Why can’t LLMs handle deduplication?

LLMs lack the precision for enterprise SQL. They hallucinate JOINs, ignore NULLs, and fail at probabilistic matching. Spider2-SQL proved 89.9% of models flunk these tasks. Deduplication requires 100% accuracy. LLMs deliver 10-17%.

How much does bad deduplication cost?

Brands overcount conversions by 40-60% with LLM-based tools. For a €1M/month budget, that’s €400K-€600K in misattributed spend.

What’s the alternative to LLM deduplication?

Causal inference. Behavioral graphs map users across devices. Probabilistic matching resolves conflicts. Incremental validation tests rules against holdout groups. No hallucinations. No rule decay.

Stop Counting the Same Sale Twice

Your attribution tool is lying to you. Not maliciously. Incompetently. LLMs can’t deduplicate conversion data. That’s a fact, not an opinion. The question is: How much is it costing you?

See how Causality Engine fixes deduplication for beauty brands. Or keep paying Meta for sales you already credited to Google. Your call.

Sources and Further Reading

• arXiv Machine Learning Papers
• Google AI Blog
• Causality Engine Resources

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