Many 486 PCs (old) and all Pentiums (or the like) should have modern 16550As (usually called just 16550's) with FIFOs. If you have something really old (pre 1990), the chip may unplug so that you may be able to upgrade by buying a 16550A chip and replacing your existing 16450 UART. If the functionality has been built into another type of chip, you are out of luck. If the UART is socketed, then upgrading is easy (if you can find a replacement). The new and old are pin-to-pin compatible. It may be more feasible to just buy a new serial card on the Internet (few retail stores stock them today) or find a used one.
Modern kernels should not allow the opening of ports with the same IO address. But one may probe for ports that are not open. If two ports have the same IO address then probing it will erroneously indicate only one port. Plug-and-play detection will find both ports so this should only be a problem if at least one port is not plug-and-play. All sorts of errors may be reported/observed for devices illegally "sharing" an IO address. See Probing.
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