In this paper, we identify new partial data redundancy among multiple cache lines that are not exploited by traditional memory compression or memory deduplication. We propose Base and Compressed Difference (BCD) deduplication that effectively utilizes the partial matches among cache lines through a novel combination of compression and deduplication to increase the effective capacity of main memory. Experimental results show that BCD achieves the average compression ratio of 1.94× for SPEC2017, DaCapo, TPC-DS, and TPC-H, which is 48.4% higher than the best prior work. We also present an efficient implementation of BCD in a modern memory hierarchy, which compresses data in both the last-level cache (LLC) and main memory with modest area overhead. Even with additional meta-data accesses and compression/deduplication operations, cycle-level simulations show that BCD improves the performance of the SPEC2017 benchmarks by 2.7% on average because it increases the effective capacity of the LLC. Overall, the results show that BCD can significantly increase the capacity of main memory with little performance overhead.