AA-的 reduplication of 浓 ('thick/concentrated'). The reduplicated form is more vivid and emotional than plain 浓的, it suggests sensory richness (a smell that fills the room, a feeling that fills the heart). The 的 is required when modifying a noun.
usage
Pairs naturally with smells (浓浓的香味), drinks (浓浓的咖啡), atmospheres (浓浓的年味 'strong New Year flavor'), and emotions (浓浓的爱意 'deep love'). Less common with literal physical thickness, for that, plain 浓 or 厚 is more typical.
