Used to play aoc before, and im pretty sure i had the same hotkey for those actions.

Like I would press B then A to build barracks, and just A to go to barracks.

But now im trying to set the hotkeys like this and they cancel each other.

Just realised I cannot set A for "go to barracks" and "create archer" on archery range, I swear i had "go to barracks", "build barracks" and "create archer" all these 3 actions under the hotkey A

Maybe it changed with last userpack 1.5, but as far as I know, you can never use a "go to xxx" hotkey also for something else (except if you add shift/ctrl/alt with it)...

I have "go to Barracks" on hotkey "I"
I have "build a barracks" on hotkey "space"+ "shift-I"
(space = for giving build order off course)

Make sure you have userpatch 1.5 selected as the patch when editing. It will still give you the warning and say that another hotkey has been cancelled, but that won't actually be the case, you can put as many things on one hotkey as you like.

If you have a hotkey for "go to barracks", it should work in every context (it doesn't matter what unit you have selected or what actions you did previously), so it cannot overlap with any other hotkey. Usually "ctrl+something" is used for this kind of actions.
On the other hand, the hotkey for "build barrack" only needs to work after the command "build" (by default "b" or "v"), and the hotkey for create archer only needs to work when an archery range is selected. This contexts are mutually exclusive so both actions could share the same hotkey.
By default you use "ctrl+B" to go to barracks, and "build then B" to build a barracks. The character is the same for each building but "ctrl" makes the difference. I use a similar pattern for common buildings.
Probably the pros used to prefer to go to important buildings without ctrl, for example with "A", but then the "A" cannot be used to any other action. Currently, MQ is used and then the more common method is to go to buildings through control groups (numbers) and then a good hotkey for going to buildings is not so important. Without MQ it is necessary to traverse buildings of some type through a hotkey (or mouse clicking...).