If you follow the news about the US-led war against al-Qaeda in the Arabian Peninsula (AQAP), you're probably familiar with the phenomenon of the zombie mujahid. In several cases, US or Yemeni officials have announced the death of certain AQAP operatives, only to have the men in question appear alive and well weeks or months later. A recent study by the UK-based NGO Reprieve looked at this issue, and also tried to figure out who the US had actually killed in place of the intended targets. Reprieve released a report based on the study on Monday. Among their key findings, Reprieve identified 41 individuals who appear to be US-designated "high-value targets," and who have each been reported killed multiple times. "Each was targeted and/or reported killed more than three times on average before they were actually killed. In one instance, a person was targeted seven times before eventually being killed. Two others were killed six times and one is believed to still be alive today."
Reprieve staff attorney Jennifer Gibson collected data on US air strikes in Yemen and Pakistan for the study, which also found that:
Strikes targeting the above  individuals killed on average 28 other people each before they actually succeeded in killing their target. In total, as many as 1,147 people may have been killed during attempts to kill just these 41 men, accounting for a quarter of all possible drone strike casualties. Yet, evidence suggests that despite multiple attempts, at least seven of these forty-one men are likely still alive and a further individual died not from drone strikes but rather natural causes.
Seventeen of the high-value targets the study identifies were/are in Yemen. According to Reprieve:
Missile strikes on these men killed 273 other people and accounted for almost half of all confirmed civilian casualties and 100% of all recorded child deaths. Each [of the 17 HVTs] was killed on average well over three times each. Strikes against these 17 targets accounted for almost half of all confirmed civilian casualties in Yemen. Yet, evidence suggests that at least four of these 17 men are still alive (Qassim al-Raimi, Nasser Abdul Karim al-Wuhayshi, Ibrahim al-Asiri, and Abdulraouf al-Dahab).
Of course, reliable data on US air strikes are hard to come by, since the targeted killing program is covert. In particular, it's hard for researchers to determine who is a civilian and who is a "militant." You can read the methodology section of the report to get a sense of how Reprieve arrived at these numbers, and how they established who was killed in which strikes. Reprieve's press release, with a link to a PDF of the full report, is here.