I'm a PPL student and when I got 15~16 hours under me I doubled up on my Saturday lessons to get me over the Solo flight; one in the morning and one in the afternoon.
The thinking here was to condense the time between pre-solo lessons (efato - go around - SaSAaA - circuit improvement) to embed these learning points and to double up the good weather opportunities where the CFI and myself were comfortable that the time was right for me to go solo.
After 5 double (solo on the 4th) Saturdays I've now cancelled the remainder of my "double" appointments and retreated back to a single lesson per day for reasons of cost and attention span. I simply didn't feel I was get the right amount of learning for the cost of the 2nd hour on some of the days. "Human Performance and Limitations".
It can be annoying when you have 2 to 3 of weeks break between lessons due to clash of availability etc. but these breaks serve a financial purpose for me as well.