had been amassing
-había estado acumulando
Past perfect progressiveconjugation ofamass.There are other translations for this conjugation.

amass

His collection, which he had been amassing throughout his lifetime, has about 10,000 pieces.
Se calcula que su colección se componía de unas 10.000 piezas, que había ido atesorando a lo largo de toda su vida.
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