The first Duplo bricks, with 2 rows of 4 knobs, appeared in 1969. The bricks were available in four colors: red, yellow, blue, and white. The knobs stick out less than current Duplo bricks, and the holes had four tags for a better grip with traditional Lego bricks. Set number 510 had 9 Duplo bricks mixed with 8 Lego bricks, while set 511 had 19 Duplo bricks and 8 Lego bricks. Both sets were designed to target younger children. The number 510 has been reused to name other Lego sets. The following year, two more sets were added with blue and red wheel plates with 4 rows of 8 knobs. In the product catalog for 1971, the sets were targeted at children from 1 to 2 years, but still sold mixed with Lego bricks, normally designed for ages 3 to 12. In 1972, the Duplo brick with 2 rows of 2 knobs was introduced and continues to be included in sets today.
In 1975 Duplo became its own product, with five sets made up exclusively of Duplo bricks. New additions included a round-topped 2 by 2 knob brick and a small 4-wheel wagon with two rows of 6 knobs. This wagon, and its coupler system, is still sold today. With these new Duplo sets, Lego is targeting children 1½ years old with the idea that when the children reach 3 the Duplo bricks can be used together with Lego bricks.
In 1977 the Duplo name was dropped in favor of Lego Preschool. Small figures the size of 2 by 2 knobs were introduced as well. They included a cylindrical head and a tapered body without any limbs (similar in design to Fisher-Price's Little People). Another new brick was a half arch. The new sets included figures, doors, and 2 by 6 knob wagons that could act as a car or train.
The name Duplo was brought back in 1979 along with a new Duplo logo with a rabbit. In 1983, other Duplo figures appeared, often called Duplo people. These figures have a moveable head, arms, and legs and look like large Lego minifigures, but the Duplo people cannot be taken apart for safer handling by small children. Also in 1983, set number 2700 was introduced with a model of a steam engine with two train cars.
In 1986, a Duplo doll house with sliding doors was introduced. This included a Duplo people mother, father, and smaller child.
In 1992, Duplo Toolo was introduced. These used internal screws to stay together.
1993 brought a grey rail train system with a stop and start track. Later, two more train systems arrived. As of 2007, Lego sells Duplo Trains trains in Thomas the Train sets in addition to two motorized sets.
The name Duplo was dropped again in favor of Explore in 2002. In the 2004 spring catalog there was a reminder that Duplo was now called Explore, but that fall the well known Duplo name was back yet again with a new rabbit logo.
What's it worth? Take a look at this Duplo price guide: sold listings for a value indication.