Multi-accounting is the operation of several accounts by the same individual or a coordinated group. The most common goal is repeatedly capturing welcome offers, but it also appears in arbitrage and in attempts to bypass limits.
Detection is where operations most often err on both sides at once. Strong, obvious signals, same IP, same device, have a high false-positive rate in shared-connection contexts, borrowed phones and families at one address.
What works better is combining weak signals: betting-pattern similarity across accounts, session overlap with alternating actions, payment-instrument and withdrawal-destination graphs, signup timing sequence, and simultaneous reaction to an offer change.
Two practices greatly reduce the cost of error: limit instead of block (remove promotion eligibility and review at withdrawal, without preventing play), and measure the false-positive rate by sampling, with blind manual review of a random batch of flagged accounts.