Thumma commonly introduces sequence with a measure of separation. Placed between the two warnings, it marks the renewed admonition as a distinct stage rather than an immediate echo. That interval can increase the seriousness with which the second occurrence is received.
It would be insufficient merely to note that the two sentences contain the same words; their discursive ordering is itself meaningful.
A close reading therefore considers the repeated assertion, the separation introduced by thumma, and the cumulative effect of the warning together.