Ca2+ + C2O42- -> CaC2O4 -> Ca2+ + C2O42-
The second reaction, dissolution, should take place as well as its counterpart, precipitation, according to the value of Ksp.
In the exercise proposed, anyhow, the quantity of ions at the beginning is so large, that it makes the common ion effect paramount.
To sum up, dissolution, actually, does not occur, because the number of moles produced by dissolution is etremely inferior, compared to those introduced at the beginning of the experiment.
Ca2+ + C2O42- CaC2O4 C2O42-
0.0050 mol + 0.0051 mol -> 0.0050 mol + 0.0001mol (surplus of C2O42- )
In case you'd have to calculate solubility, pay attention: you must consider a common ion again.
I hope to have been clearer this time.