There will be an automation triggered for every linked entity, and if this automation’s action is to update a to-one relation with the linked item, any secondary automation which is triggered by this to-one relation being updated will also be triggered twice.
But any secondary automations will sit in the queue waiting to execute, and by the time they run, the update value of the to-one field may not be what you expect.
As an example, imagine this sequence:
| To many | To one |
|---|---|
| empty | empty |
User links two things
| To many | To one |
|---|---|
| ThingA, ThingB | empty |
Automation 1 (that triggers on linked item) is triggered twice.
First execution takes place - it updates the to-one field
| To many | To one |
|---|---|
| ThingA, ThingB | ThingA |
Automation 2 (which triggers on the to-one field being updated) is triggered, but will only execute when the queue of automations is empty
Second execution of automation 1 takes place.
| To many | To one |
|---|---|
| ThingA, ThingB | ThingB |
Automation 2 (which triggers on the to-one field being updated) is triggered again, since the value of the to-one field has changed.
The first execution of automation 2 takes place, and sees that the to-one field contains the value ‘ThingB’.
The second execution of automation 2 takes place, and sees that the to-one field contains the value ‘ThingB’.
Ideally, yes, but as @YvetteLans has discovered, markdown does not support referencing [Step 1 linked item] unlike formulas, so I can’t think of a way of making a comment that references the linked item all within a single automation rule.