There are two ways in which a person can fail to deserve what they have. The first is that they are actually undeserving of it: the prodigal son does not deserve his father’s welcome, Job did not deserve to be tormented with destruction and agony. The second is that the concept of desert fails to apply: thus neither James Potter nor Lily Evans deserved the love of Lily Evans, because in the decision of who she should marry desert is simply not a relevant factor.
These two situations are very different, yet we use the same phrase of “not deserving” to describe them both. This is liable to create dangerous confusion: when a good (or bad) is appropriate for distribution by deservingness, someone’s lack of desert generally provides a reason for taking that good away from them (and typically giving to them). Physical property is, in most naive views of the world, taken to be appropriate for distribution according to desert: thus a simple argument for economic redistribution would be that the poor are no less deserving than are the rich of worldly goods.
When a good is not appropriate for distribution according to desert - for example, love - the fact that someone is undeserving is no reason to remove the good from them. While most people naively think of private property as something to be distributed according to desert, this view is exceedingly rare among philosophers. The most obvious example of an anti-desert theorist is John Rawls, who argued that we cannot deserve anything at all: any good traits we possess are the results either of our environment or of our genes, neither of which we chose and therefore neither of which we can be credited for.
This anti-realism about desert does not - cannot - provide an argument for redistribution of goods. If desert is not real, then no goods can be appropriately distributed according to desert, and so the fact that the rich are no more deserving than the poor is no argument for redistribution. One may, of course, favour redistribution on other grounds, and this was Rawls’ purpose: to disarm desert-based arguments against redistribution! But if one only takes the conclusion of his argument - that the rich do not deserve their wealth - and puts it not into the context of Rawls’ wider theory, but rather the naive view that desert is real and is a moral basis for property, then one arrives at a rhetorically effective, but subtly self-contradictory, agument for redistribution. I suspect that many people who dabble in political philosophy without studying it in depth, including many politics undergrads ae liable to fall into this trap.