Objective: Reapportionment and the attendant process of redistricting cause instances where incumbents are faced with the decision to retire or run against a congressional colleague. We investigate two major questions surrounding the previously understudied population of dueling incumbent primaries: First, under what conditions is a dueling incumbent primary likely to manifest? Second, how are dueling incumbent primaries different from the typical primary featuring a single incumbent? Methods: We address these questions using a novel dataset aggregating descriptive congressional district information on these contests from 1962 to 2016, and separately for 2022. Results: Our findings show these contests primarily emerge via electoral retrenchment, meaning a state loses representation in reapportionment. Dueling incumbent primaries also arise from the strategic calculations of partisan line drawers. Conclusions: Not only do dueling incumbent primaries comprise a disproportionate share of incumbent defeats, but compared to single incumbent contests, primary duels feature very high redrawn constituencies, voters new to these incumbents, and this reality contributes to the high-risk/reward nature of these rare but electorally consequential contests.Redistricting can be a geographical nightmare, especially for some incumbents contemplating where to run for another term (Fenno 1978, pp. 11-12). The degree to which congressional boundaries are altered has a direct bearing on the decisions officeholders make with respect to their most likely chance of winning reelection (Cain 1984). For the lion's share of incumbents, changes to districts in a given election cycle are not drastic enough to warrant much strategic contemplation. Simply put, most incumbents have the luxury of running in a district only marginally altered from the one they represented prior to redistricting, and in most circumstances, they enjoy a very comfortable probability of reelection.However, due to redistricting, some incumbents find themselves running in a primary against a fellow incumbent. We label such contests "dueling incumbent primaries." Heretofore, no published studies have comprehensively catalogued and examined this notable subset of incumbents who must first prevail against a colleague to prolong their congressional tenure. 1 This article is motivated by the dearth of systematic research about dueling incumbent House primaries. In this vein, we first establish the electoral significance and prevalence of these contests dating back to 1962, the year the equal district population 1 See Ashton,Crespin, and McKee (2022) for an analysis of dueling incumbent general elections.