Sometimes there are Weekly Tasks that require a Sim to start a career (e.g., Start a career as an Athlete) or have or have x Sims be in that career. When I get such tasks, I will have an unemployed Sim join the career and then immediately quit the career.
I don't see a reason why a player would have more than 1 Sim on the same career (e.g., 2 Scientists) because I don't find any practical use in The Sim FreePlay to have 2+ Politicians, Athletes, etc. especially if using Competition Center where it's only useful for 1 Sim anyway and FreePlay also doesn't have unique Sim specific rewards like other games have. In The Sims 4, it's usually common to have 2 more Sims in the same career if the player wants to unlock objects in both Branches or if more than 1 Sim is completing an Aspiration that requires a Sim to reach an appropriate level of the career (e.g., Culinary).
Similar with the "Buy an object" Task, they often appear when it's not necessary such as being asked to buy a Computer when you already have 10 of them. Same goes for the Careers, do you really need 4 Artists? I think one is enough.
Origin ID: GothicSimmer, YouTube: