Bank and bank group conflicts are major performance bottlenecks for memory intensive workloads. Idealized experiments show removing bank and bank group conflicts collectively can improve performance by up to 37.5% and by 22.5% on average for our mix of multi-programmed memory intensive workloads. We propose the Duplicon Cache to mitigate bank and bank group conflict penalties by duplicating select lines of data to an alternative bank group, giving the memory controller the freedom to source the data from the bank group which avoids conflicts. The Duplicon Cache is entirely implemented in the memory controller and does not require changes to commodity memory. We identify and address the main challenges associated with duplication: 1) tracking duplicated data efficiently, 2) identifying which data to duplicate, and 3) replacing stale duplicated data while protecting useful ones. Our evaluations show the Duplicon Cache configured with 128MB of storage (out of 16GB of main memory) improves performance by 8.3% while reducing energy by 5.6%.