...are at those addresses , reported a overlap.
If Yo do not have "ORG xxx " directive , or xxx is 0 , then these messages are "normal".
Declare ORG 0100h at least.

Probably linker searches for labels as "Reset", "Timer1_irq" and so on.
Its good time to study where and how "Interrupt Service Routines" are implemented in MCS51.

I think - in Your case Keil is helping to learn.
First - read how segments (DATA,BIT,IDATA,XDATA,CODE) should be invoked by You in Your source. Second - how to reserve RAM space,

MYBITS1_8: ds 8
...
MYbytes1:12: ds 12

this usually works.


Third - find 'fixed' names in documentation, or see examples - i suppose somewhere in exmples You will find something like:

public AT_RESET_ENTRY
....
AT_RESET_ENTRY:
ljmp mainstart



O, linker should report somewhere such "fixed" unresolved labels.