number being doubled when saved

[orchid pilot]

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[scenic nymph]

Documentation suggestion for @tired heron:
Addition assignment (+=)
The addition assignment (+=) operator performs addition (which is either numeric addition or string concatenation) on the two operands and assigns the result to the left operand.

[manic granite]

x += y is equivalent to x = x + y, except that the expression x is only evaluated once.

for it doubling the number, no idea