What If you receive Leave Encashment from more than one employer?

The proviso to section 10(10AA) provides as under
Provided further that where any such payment or payments was or were received in any one or more earlier previous years also and the whole or any part of the amount of such payment or payments was or were not included in the total income of the assessed of such previous year or years, the amount exempt from income-tax under this sub-clause shall not exceed the limit so specified, as reduced by the amount or, as the case may be, the aggregate amount not included in the total income of any such previous year or years.
The aforesaid provision, in simple terms states that total exemption through out your career is only unto maximum Rest 3, 00,000. So , if you have already claimed certain amount as exempt , that much amount will be reduced from the income compute for exemption purpose u/s 10(10AA) of the I T Act.