I wouldn't actually longer term trade doubles using AIM or AIM like method.
About the only thing doubles may have been useful for IMO is if you bought the double long with half the amount that you would have normally bought stock with and potentially saved on trading costs. But in practice their fees and costs mitigate any potential benefits on that front.
The only other potential use for doubles is for shorter term trading and where you don't have access to credit based trading.
If say you want to protect a collective set of long positions via an Index short to around the same capital amount as the collective long positions held, then the double could enable you to do so (up to twice the available amount of cash reserves you had at that time).
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