One plus one equals ONE
1 + 1 = 1

You would measure the volume (let’s say 0.001 cubic mm). Adding the two would get 0.002 cubic mm, not 0.001 mm. Vague concepts can be manipulated, but not solid measurements.

let a=b
multiply both sides by a : aa=ba
a^2 = ba
substract b^2 from both sides : a^2 - b^2 = ba - b^2
factor both sides : (a - b)(a + b) = b (a - b)
divide both sides by (a - b) : (a + b) = b
substitube a=b=1 : 1 + 1 = 1
if 1 raindrop combines with another raindrop, it is still 1 raindrop