Weight based price calculation applies twice on initial additional weight cost

Hello, Think this is a simple question
I have created two rules for weight based calculations:
(Prices in UK pounds)

1) Standard postage rate for orders up to 1Kg
Set flat rate to 3, Set Order weight to 1Kg or equal
This works perfectly

2) Rule 2 is flat rate plus 0.50 for each additional 1Kg
Set flat rate to 3, Set Order Weight set to above 1Kg
Set Charge 0.50 for each additional 1Kg

I created a product that weighed 0.5Kg and tested

Problem is:
3 items (1.5Kg) adds an additional 1 not 0.50 to the flat rate.
4 items (2Kg) adds an additional 1 not 0.50 to the flat rate.

Thereafter adding items does calculate properly:
5 items (2.5Kg) shows 1.5 and the flat rate and
6 items (3Kg) shows 1.5 and the flat rate
7 items (3.5Kg) shows 2 and the flat rate, and so it continues to add the correct wight/cost properly.

Why does the initial calculation double the set cost, I guess i must be doing or setting something wrong?

Any help appreciated please.

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