Double Impact, sometimes know as the E1 Replacements is a mod for DooM, meant to be a replacement for Knee-Deep in the Dead, the first episode from DooM, developed by Ralph "Ralphis" Vickers and Matt "Rottking" Cibulas that was released on November 30th, 2011. It features larger combat arenas, more emphasis on exploration, more "space-like" views, and more dramatic lighting effects. Other than a few new musical tracks, it is completely vanilla. It received a Cacoward in 2011. On 11 November 2013, Double Impact was included with Freedoom: Phase 1 as part of the fourth chapter of that game.
On 13th February 2020, Double Impact was the first community-made mod to be adopted by Bethesda as one of the official add-ons for the Doom Classic Unity port. This version is slightly different. First, the intermission image, which incorporates the cover art from the unlicensed Hungarian novel Doom - Tisztítótűz, was recreated using instead the Eisen character from Quake Champions (specifically, the Eisen skin variant featuring a cap and an eyepatch). Another difference involves the alteration of the d_e1m8 theme to remove references to the theme of Doom 64's main menu.
- The music track for E1M2: Central Computing is a mash-up of the tracks "Dark Halls," "Suspense," and "Demons from Adrian's Pen"
- The music track for E1M8: Launch Bay is a remix of the track "Phobos Anomaly"