I have the following dataset with 20 million rows. It’s data on companies and user by month.
I have created first_app_company, which flags first appeareance of a company in the dataset. The code is as follows

df$first_app_company <- as.numeric(!duplicated(df$Company_id))

Company_id    Customer   Month-Year          first_app_company

11               X             201501               1
12               Y             201501               1
13               Z             201501               1
13               Q             201501               0
13               R             201501               0
14               E             201501               1
14               W             201501               0 
15               X             201501               1 
15               Z             201501               0 
15               H             201501               0
15               K             201501               0
16               Q             201501               0

However, I realised now, that when a company enters my dataset in month M, I would like to flag all rows that match that company name and month M as 1. So my desired output would look like:
(Please note that 201501 is first month in my dataset so all entries will be flagged as 1, but it shows the logic)

Company_id    Customer   Month-Year          first_app_company

11               X             201501               1
12               Y             201501               1
13               Z             201501               1
13               Q             201501               1
13               R             201501               1
14               E             201501               1
14               W             201501               1 
15               X             201501               1 
15               Z             201501               1 
15               H             201501               1
15               K             201501               1
16               Q             201501               1

I am currently trying to figure it out with lead and lag functions from base R, but it is getting a bit confusing. Was hoping someone would point me in the right direction