We examined the role of L1 writing on L2 writing performance by repeating the same topic twice through L1 writing first and L2 writing later. We designed what we call a 'L1 + L2 writing task' and a 'L2-only writing task' respectively. In the L1 + L2 writing task, students wrote a text in their L1, and wrote the same topic in L2, by removing the initial L1 essay. We focused on validating which task conditions (± L1 writing) account for the variation in linguistic performance in EFL high-school students' writing. Our study was conducted with two intact classes (n = 60) assigned to the L2-only, and students from the other two classes (n = 60) assigned to the L1 + L2. The findings indicate that L1 writing may push students to direct their attentional resources toward effective text construction, thereby mitigating a large number of simultaneous demands on attention. We add to empirical knowledge on the effects of L1 writing in task repetition, by exploring how task repetition affected our students' L2 writing performance.