| Metformin reduces the liver's output of glucose. That output is elevated above normal in those with insulin resistance. Also the liver will stop producing glucose when it senses high concentrations of insulin released from the pancreas. Injected insulin does not "signal" the liver in the same way.
There is no contradiction in both taking meformin and injecting(/pumping) insulin.
Specifically,
* type 2s injecting insulin (eventually most T2s do),
* type 1s with insulin resistance, this "double diabetes" is rather common, and
* type 1s who otherwise can not control their Dawn Phenomenon
can profitably use metformin, if they do not have kidney problems. | 
