A source volume and a copy volume are stored, where the copy volume contains data that has been modified by writes to the source volume. In response to a write to a particular location and resource volume, a deduplication data structure is checked to determine whether the copy volume contains data that is the same as data in the particular location. In response to determining from the deduplication data structure that a copy volume contains the same data as the data in the particular location, a reference to the first location of the copy volume storing the same data is added instead of repeating storing of the data of the particular location in the copy volume.