@crookid the 15$ board from TL is amplifier for PT100 that should be a copy of the ultimaker PT100 amplifier.
On the marlin you would connect the output directly to a analog input pin (no 4k7 voltage divider resistor) and you would set the RTD type to 20. If you look at Marlin source:
https://github.com/MarlinFirmware/Marlin/blob/57ca996c31e39b6935e2a24451040fa4379dc23e/Marlin/Configuration.h#L515
you see that
* 20 : Pt100 with circuit in the Ultimainboard V2.x with mainboard ADC reference voltage = INA826 amplifier-board supply voltage.
* NOTE: (1) Must use an ADC input with no pullup. (2) Some INA826 amplifiers are unreliable at 3.3V so consider using sensor 147, 110, or 21.
so, in order to use this amp board (from triangle labs or from ultimaker or ..) you need clear analog input pin on the DUET.
If we look at the duet schematic:
the thermistor0,1,2 have 4k7 to advref (pullup/voltage divider) so you cannot use them, but you should be able to use any of the thermistor4-7 pins to feed in the output of that amplifier board.
That should work wrt how to connect. Now, how to configure PT100 amp in the firmware to show proper temp values - I have no clue, someone else will have to step in