It means in 2 or more different locations your code, you made 'i_user_data_address' = to 1 value, then another.
Or you made an = assignment and also somewhere else in logic a <= to the reg 'i_user_data_address'.
Or, you are making i_user_data_address <= to a value and have it also tied to a sub-module's .xxx(i_user_data_address) who is set to an output, ie that module is setting i_user_data_address to a value while in you top modules main code, you are also making it <= to a value as well. IE, 2 drivers to the same reg.
Last case may have to do with making the reg <= a value in 1 clock domain while simultaneously making it <= to another value in another clock domain.