What happens on the STM32 when two pins are configured to have the same alternate function?
On processors which use per-pin registers to select I/O functions, it is generally possible to route an output function to multiple pins without conflict; all pins will echo the same output.  The effect of having multiple pins connected to an input is often unspecified; if e.g. a UART had I/O pins 3 and 4 connected to a UART, it might behave as though the UART was connected to an "AND" gate which took pins 3 and 4 as inputs, or it might behave as though it was connected to one pin and ignore the other, or it might connect both pins to the UART's input buffer through transistors that had a moderate amount of resistance, or it might draw extra current when pin 3 is high and 4 is low or vice versa, or it might do just about anything else imaginable.  I don't recall having seen any particular guarantees that any of the STM32 family chips I looked at would favor any particular approach over any other.

A somewhat nicer design approach, used in some Microchip parts (perhaps some STM parts as well) is to have each I/O function include a multiplexer to select which pin it should accept input from, and have pin include a multiplexer to indicate the I/O function from which it should output data.  Such a design makes it possible to have one pin feed multiple I/O functions, while simultaneously eliminating any ambiguities posed by conflicting configurations.