The plosive alliteration in "deal double", the repeated "d" sound, injects a belligerent sense of urgency into the Nurse's speech, which establishes a tumultuous energy surrounding Romeo and Juliet's secret union. It creates a sense of verbal confrontation, despite the Nurse's lower-class status, and serves as a momentary equilibrium between impassioned love and rational caution.
This phrase presents the Nurse as a benevolent protector of Juliet, despite her informal register, where the alliteration compels Romeo, and the audience, to recognise the stakes of duplicity, especially when directed towards someone as callow as Juliet.
The Nurse serves as an epitome of maternal benevolence through her relationship with Juliet, striving to protect her from any potential pain Romeo may cause her.