Regardless of whether you believe no insulin got into him, we recommend you never give a 2nd shot of the long acting insulin, because if you are mistaken, the cat might go into hypoglycemia (too low glucose) which can be fatal.
An option for a cat that has experienced ketones or DKA is to check around the nadir period, then if the glucose is very high, with very careful coaching from folks who have done it and if you are home to monitor, you might use a shorter acting insulin (NPH) to cover the next 6 hours or so until the regular shot time. NPH hits harder and faster than ProZinc so initially you start with teeny, tiny doses until you learn what works for your cat.
"Better too high for a day than too low for a moment."