The laws of physics are still applicable to our world. Therefore, we have to use frequency separation. It means that TX frequency should be at least 3 MHz away from RX frequency on the repeaters. The recommended value is 5 MHz away.
If you have multiple repeaters in same area (e.g. a site of Capacity Plus), all TX frequencies can be close by (about 100 kHz separation), but the TX frequencies area should be at least 3 MHz away from RX frequencies area. If you are going to use analog radios, those should be separated from both RX and TX areas by another 3-5 MHz to exclude the interference.
When TX frequencies are too close to RX ones, the repeater sometimes can have the interference between its own antennas, which will cause the loopback and decrease in communication quality or even a repeater malfunction. When analog radios are too close to digital frequencies, they will hear digital buzzing during digital transmissions and can affect the repeater functioning, just as any radio noise.
Do not neglect the laws of physics! 🙂