Double taxation is a question to investigate, not a conclusion drawn from two tax bills. Establish who earned the income, its legal character and the period to which each country's tax relates. Spanish domestic rules and the relevant tax treaty may provide different mechanisms. The existence of a treaty does not mean that every foreign payment receives an automatic exemption.
Begin with a gross income reconciliation. Show what was earned before foreign withholding, what reached the bank and what appeared on each return. Currency conversions and different tax years can create apparent differences that are not additional income. Keep the exchange rate methodology visible and identify the source of each figure so the calculation can be checked later.
Obtain foreign returns, assessment notices, payment confirmations and withholding certificates. Distinguish a final tax charge from a provisional deduction that could still be refunded. Record whether the foreign authority has amended the position. A payment description in a bank statement may not demonstrate the nature of the tax or whether it belongs to the same person and income.
For a hypothetical consultant resident in Spain, an overseas customer withholds tax from a fee. The consultant also incurs tax on that income in Spain. Before calculating relief, the adviser checks the service contract, where the activity occurred, the treaty category and whether the foreign withholding was properly imposed. Matching amounts alone cannot answer those legal questions reliably.
The treaty review should identify the actual agreement between the countries and the relevant income article. Residence, beneficial entitlement and any other applicable conditions need evidence. Where relief is available, its calculation may be limited; do not treat all foreign taxes as an unrestricted credit against any Spanish liability. Social contributions and indirect taxes also require separate consideration rather than casual inclusion.
Ask for a short written bridge between the documents and the proposed Spanish treatment. It should explain the relief mechanism, calculation basis, unresolved foreign refunds and assumptions requiring confirmation. Bring discrepancies forward early instead of adjusting figures merely to match a certificate. A targeted crossborder review is most useful when both countries' records are available and the question concerns a clearly identified income stream.