Washing out the cup under the chuppah

Hi,

Normally, a cup used to hold wine [or grape juice] for a special mitzvah [kos shel berachah] must be washed out, as a pre-requisite for the mitzvah. However, under the chuppah at a wedding it is common to use the same cup for both the initial kiddushin stage and then again for the sheva berachot of nisuin, adding wine in between. It may not be necessary to wash out the cup in between, because wine is intentionally left in the cup after the first berachah, for use with the second berachah, and so it has not lost the identity associated with the initial washing.

(Rivivot Ephraim 1:150:5)

Have a great day,
Mordechai