duplicate, double, compound, multiple
複 centers on things that are not single: something doubled, duplicated, or made of multiple parts. The idea of a compound or complex whole grows naturally from that sense of layered multiplicity.
複 combines 衤 (clothing) with 复, which likely contributes both sound and the idea of doubling or layering. The character originally referred to lined or double-layered garments, and the broader sense of duplication developed from there.
The clothing radical 衤 beside 复 suggests layers of fabric folded over each other. Picture a double-layered coat: 複 is about duplication and multiple layers.
For フク, imagine a fuku (服, clothing) that is double-layered: fuku -> フク, and the doubled clothing matches the kanji's meaning of duplicate.
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plural; multiple
complex; complicated; intricate; mixed (feelings)
duplication; repetition; overlapping; redundancy; restoration
composite; combined; complex
compound; composite; multiple; re-; bi-
reproduction; duplication; reprinting
copying; duplication; reproduction; copy
complex number
complex
multitude
double track; two-track line
simplicity and complexity; singular and plural; single and double; singles and doubles (in tennis)
compound eye
compound interest
double-entry (e.g. bookkeeping)
four-track rail line (two in each direction); quadruple railway track
compound leaf; multifoliate leaf
compound word; compound term
Alpine combined
diazotype; diazo