The equality trap
Matching identifier text does not establish a shared customer. Suppose a billing system assigns 0042 to Mira and an independently managed support system assigns 0042 to Leon. Joining on those digits merges different people. A nameplate labels a door within a building; the same label elsewhere need not identify that door. Customer numbers likewise need a scope. This counterexample defeats the assumption that equal strings alone justify a match, without proving that all cross-system matches are wrong.
Compare a qualified key
Define the namespace from the authority that allocates identifiers, including the tenant when numbering is tenant-specific. Compare the full tuple: issuer, tenant, local identifier. Keep these components separate or use an unambiguous encoding. Preserve leading zeros unless the issuer documents their equivalence. Different qualified keys can still refer to one person; linking them requires a verified mapping or other authoritative evidence. Namespacing prevents accidental collisions, but it does not resolve identity by itself.
A hypothetical boundary case
Imagine a migration after which an issuer reassigns 0042 from a closed account to a new account. The namespace remains unchanged, so even a qualified key collides across time. Decision rule: if identifiers can be reused, include an immutable account instance or a documented allocation generation. If neither exists, withhold automatic linking until authoritative evidence resolves the ambiguity. A timestamp helps only when the issuer guarantees how it distinguishes allocations.
Check the contract
Before changing a join, check the issuer's uniqueness, reuse, and formatting rules. Try two synthetic records with equal local identifiers and different namespaces; they should remain separate. Then try a verified mapping between different keys and a reused identifier without allocation evidence. Accept the verified link; leave the ambiguous case unresolved. Record why each decision was made so the same evidence produces the same outcome during a later review. ABOLTUS is in development. These examples are illustrative, not reported test results.