fix: avoid adding duplicate text-decoration fallback when multiple declarations exist

[lukhanina]

• Check if previous declaration has same value before adding fallback
• Skip adding -webkit prefix when following text-decoration exists
• Fixes formatting issues with consecutive text-decoration rules

[romainmenke]

@lukhanina can you check #1776
This is a much smaller fix, more inline with existing code.
I think it still covers what you intended?