From the Super Mario Wiki
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Each e-Coin features a different image on one side; the other side of each e-Coin shows the e-Reader "e". Each e-Coin can only be collected once by Mario or Luigi before displaying in the World-e Castle, where they reside in individual frames with their level of origin underneath.
There are eight frames and eight e-Coins, but they appear to serve no purpose.
e-Coins and levels