薄薄的

AAB-pattern reduplication (薄薄 + 的) softens and intensifies the adjective in vivid description. It's a stylistic choice - 一件薄外套 sounds neutral; 一件薄薄的外套 sounds warmer, more sensory, almost tender. Common in literary and conversational description alike.

mistakes

Note the reading: when 薄 is reduplicated as a colloquial descriptor it's báo (everyday "thin"); the literary reading bó (e.g. 薄弱 "flimsy") doesn't reduplicate this way.