Well, if you want it to be a one-time only event, then you'll either have to use vars or flags for that. For now, I suggest you use flags. There are scripting tutorials in the tutorials section for that.
Anyway, on A-Map, set the [Unknown] and [Var Number] to 0003 and 4000 respectively.
And then add a check and set flag commands into your script.
Flags might be a tiny bit easier to understand, but you'd be better off using a var.
Here's the thing
Var is short for Variable. A variable's purpose is to hold a value to later be checked.
For example, you have a script in which you have to choose either Charizard or Blastoise.
If you choose Charizard you give the var a value of 1.
If you choose Blastoise then you give the var a value of 2.
In a later script, you check the var too see wheter the var is at 0x1 or at 0x2
Now, in this specific case, what you do is that you set a var and a value to a script tile. If the box in amap for value has 1, and the var number 4050, the script will only run if the var 4050 has a value of 1
With that said, if you dont change the value of the var before the player steps on the script tile, the script wont happen
Now let's say that in some script you assigned 0x1 to 0x4050
Now, when you step in the tile the script will run
Now, if at the end of the script you put setvar 0x4050 0x2, the value of the var will now be 2, and the script wont happen once you step on the script tile
Hope I helped and didn't sound confusing.
(closing notes, all vars are 0 before any changed has been applied to them, and safe variables range between 4011 and 40FF)
EDIT: forgot to say, you also don't need to put 0003 in the unknown box in amap, it does the same as not putting anything