They look the same thing to me, but they serve for different pursposes.
MWRR is on CFAI V6, page 131. – SS17
MIRR is on CFAI V6, page 286. – SS18
The Modified IRR method differs from the original internal rate of return method in that the exponent is the proportion of the measurement period that each cash flow is in the portfolio. Therefore, while the original IRR is a money-weighted return, the Modified IRR approximates a time-weighted return. (Level III V6, p. 286).
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