This is a very good article which explains, in common language, the reason for the Windfall Elimination Provision (WEP). WEP may be called "double dipping" in common terms.
The article also gives an overview of how Social Security provides a better "rate of return" to low income workers.
Excerpts :
But in a nutshell, I can tell you that Maria’s $1,150 benefit represents about 90 percent of her average lifetime monthly wage. Whereas Frank’s $2,400 Social Security check is probably about 40 percent of his average monthly pre-retirement income.
Had my benefit been figured using the regular Social Security formula, I would have received the same 90 percent rate of return that my neighbor’s maid is getting. Why? Because I look like a poor person to the Social Security Administration’s computers. I’ve got only 15 years of covered earnings. The Social Security benefit formula uses a 35-year base. In other words, there are 20 years of “zero” earnings on my Social Security record. That makes me look as poor as Maria.