Place 9 cups raspberries in a blender; process until smooth. Strain mixture through a fine sieve, pressing lightly with the back of a spoon; discard seeds. Combine raspberry puree and remaining 3 cups raspberries in a large bowl; partially mash raspberries with a spoon. Stir in 1 1/2 cups sugar.
Combine 3/4 cup warm water and pectin in a small saucepan, stirring to dissolve. Stir in remaining 3/4 cup sugar; bring to a boil over medium heat, stirring constantly. Remove from heat; add pectin mixture to raspberry mixture in bowl, stirring until combined. Cool to room temperature. Mixture will thicken as it cools. Spoon into airtight containers; cover and chill 8 hours or overnight.