Informally, I might say to the lead "follow me after 3", meaning "start like I started, 3 bars later". The lead could then keep following me, duplicating me exactly, or diverge into a counterpoint.
To a technical audience, there are some precise words you can use. If the duplication is short-lived, that is the two voices start with the same melody (entering at different times) but diverge, imitation is appropriate:
The restatement in close succession of melodic figures in different voices in polyphonic textures
A polyphonic musical texture in which a melodic idea is freely or strictly echoed by successive voices. A section of freer echoing in this manner if often referred to as a "point of imitation"; Strict imitation is called "canon."
If the imitation is more rigid, to the point of being an exact duplicate in a different voice, then canon is the better term. From Wikipedia:
One melody is strictly imitated by a second part after a delay in the entrance of the second part. In order for the parts to end simultaneously, the canon may break down at the end of the composition. The canonic parts may occur at the unison or some other interval.
A familiar a capella canon is "singing in the round", like Row Row Row Your Boat.
These terms have been around since at least the Renaissance, as both canon and imitation form an important part of the sacred liturgy (eg choral mass) and secular chants (eg English ayres).