...if it can even be realized...
It can, think of a 2m repeater duplexer!
Here in the U.S. (from memory) the repeater bandplan has a pair assigned that would require the user to TX as low as 144.520MHz. That's 130KHz above the APRS frequency- well under the 600KHz spacing that duplexers are designed for. In addition, the passband of a duplexer is optimized for a small spectrum- not 145-148 MHz.
My company designs some pretty sharp notch filters in small packages- but I don't think you'll see anything with a significant notchat 144.390MHz and yet still have acceptable insertion loss for the repeater band, decent return loss and also capable of handling 50W unless it employs a COR bypass.