The LORD said to Moses,
Some observe that the repetition of those words, The Lord spoke unto Moses, here and afterwards (Exo 30:17, Exo 30:22, Exo 30:34), intimates that God did not deliver these precepts to Moses in the mount, in a continued discourse, but with many intermissions, giving him time either to write what was said to him or at least to charge his memory with it.
Commenting on Exodus 30:11-16
And the Lord spake unto Moses,.... Continued his discourse; or, there being some intermission, reassumed it: saying; as follows.
When thou takest the sum of the children of Israel, &c.--Moses did so twice, and doubtless observed the law here prescribed. The tax was not levied from women, minors, old men (Num 1:42, Num 1:45), and the Levites (Num 1:47), they being not numbered.