Never heard of this - it needs to be challenged.
Firstly, there's no such thing as "small print". Consumer insurance contracts are written in plain English and are a doddle to follow - see the link below to a Direct Line policy.
https://www.directline.com/lib/pdf/dl-home-157-0818.pdf
Secondly, following a cursory look at the policy, please see the excerpt below headed "Your excess". A pertinent part of which states "We will only take off one excess for each claim". Taking one excess off means they will deduct the excess from the settlement.
"Your excess
You will have to pay any excesses shown in your schedule. This includes the specific escape of water excess.
There is no excess for claims under the additional covers contents outside and garden plants.
We will only take off one excess for each claim, unless there is an endorsement shown in your policy schedule to say otherwise.
If we have asked one of our suppliers to deal with all or part of your claim, we may ask them to collect the excess from you."