Doubling of quantifiers is ubiquitous in natural languages. This paper explores an understudied case of exclusive focus 'only' doubling of sentence-final particles (SFPs) and adverbial particles in Cantonese, Mandarin and Vietnamese. Providing novel syntactic arguments from minimality effects and phase/island-based locality effects, it is argued that 'only' SFPs establish an Agree relation with exclusive operators. Furthermore, it is shown that SFP doubling contrasts with doubling with adfocus particles in locality effects, the latter of which involves a covert movement dependency. The findings not only strengthen the existing operator-particle approach (e.g., Quek & Hirsch 2017), but also raise interesting questions on co-existing syntactic mechanisms of doubling.