Answer to your questions resides in how you have designed the flow. Having said that...answer to your question is simple that is:
1. If you want each record to be uniquely identified, create w/o DSO since it has three keys which will segregate data. In your case both record will be in DSO 300 and 400, same will be in Cube. Cube always represents data uniquely but shows aggregated in query.
2. If you want data to be overwritten/summed then you can go for standard DSO, now you need to design DSO as per you need (important are the keys of DSO), if keys are same then in your case 400 will overwrite 300 else both will be loaded. Other user also said if you load both records at same time then 400 will overwrite 300 but if you have loaded them separately then both will appear in cube since both load will take at different time.
It is all about how you design your DSO (basically tables that you play with)....
Thanks & Regards,
Mahen