This is from a very dim and distant past of mine where I was on the packet switching side of things.
The simple messaging service records all messages for ever (or it used to). The only difference when a message has been received the message gets flagged as delivered and doesn't get sent again. If the account linked to that number gets a snag in it this can cause the sent flag to be corrupted and things to get sent multiple times. From what I could see the fix was to create a new account and then link it to the number to get rid of the corruption.
Now the problem you will have is that the first line of support will not have a clue what you are on about and trying to get through the first line will not be easy. Second line support will not be allowed or have the access rights to do whats required (and maybe also won't have a clue whats wrong). And the folk you really need to speak to and will have the rights and knowledge to sort it will have way more important things to do with the network and will never speak to customers (which if you ever meet any of them you will understand the reason why).
Proberly easy way to get it fixed would be to wait until your contract is up and transfer both numbers to pay as you go sim cards. Then shop about to decide what you want to go for next, even if its with the same company. Then transfer the numbers back to the new account. Until you break the link between that number and the current account you will proberly continue to get the texts off and on.
To show you how bad it gets I have know a OEM engineer of the telephone exchanges have to do this with his own personal number because of the same issues and even through contacts he couldn't get things sorted because of the access rights to the billing gateway.