Many questions training unbalanced and duplicated data
Repeated entry is as good as you are adding weighted samples i.e. those samples will have extra weights in the learning processes.
 So, you should look into the data and decide if that weight is valid for those samples. Otherwise, it might overfit the Model
On Oversample, Undersample
 This is a very broad question. Please do read some relevant blogs/SE posts.
 You should also try
- Both, Over-sampling and Under-sampling together i.e. Hybrid approach
- Adding appropriate class-weight to the learning i.e. Cost-sensitive learning
You may start here -
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