Why doesn't self condensation occur in acetoacetic ester synthesis
Self-condensation is typically not an issue with 1,3-dicarbonyl compounds, such as the one in your question. Why? Due to their acidity ($\mathrm{p}K_\mathrm{a}$ around 9 to 11), most 1,3-dicarbonyls exist in their enolate form in solution. Therefore, there is very little of the original electrophilic 1,3-dicarbonyl which the enolate can attack. Hence, the amount of self-condensation taking place is relatively little and usually not a concern.