The currency symbol handling of the custom amount fields in banners on enwiki is buggy.

Steps to reproduce

• Call a banner on enwiki (e. g. https://en.wikipedia.org/?banner=B19_WMDE_EN_Desktop_02_ctrl)
• Enter a value into the custom amount field.
• Click anywhere to make the field lose the input focus.
• Click the field again and change its value without removing the currency symbol that was automatically added.

Expected behaviour

• There is only one currency character in the field.

Actual behaviour

• Although there already is one, another Euro character is added to the field.