certificate validation failure from trustore having more than one public certificate with same Distinguished name?

I have a scenario where the customer public certificate is going to get expired, so he created a new public certificate with the same DN. Upon validating the user certificates which are generated using the customer's certificate using the client certificate key. so, when the trustore has both the public key, it only validates the one generates with older public key not with the new public key. for others it throws an error:

Caused by: sun.security.validator.ValidatorException: Certificate signature validation failed
    at java.base/sun.security.validator.SimpleValidator.engineValidate(Unknown Source)
    at java.base/sun.security.validator.Validator.validate(Unknown Source)
    at java.base/sun.security.ssl.X509TrustManagerImpl.validate(Unknown Source)
    at java.base/sun.security.ssl.X509TrustManagerImpl.checkTrusted(Unknown Source)
    at java.base/sun.security.ssl.X509TrustManagerImpl.checkClientTrusted(Unknown Source)
    at io.netty.handler.ssl.ReferenceCountedOpenSslServerContext$ExtendedTrustManagerVerifyCallback.verify(ReferenceCountedOpenSslServerContext.java:264)
    at io.netty.handler.ssl.ReferenceCountedOpenSslContext$AbstractCertificateVerifier.verify(ReferenceCountedOpenSslContext.java:700)
    at io.netty.internal.tcnative.SSL.readFromSSL(Native Method)
    at io.netty.handler.ssl.ReferenceCountedOpenSslEngine.readPlaintextData(ReferenceCountedOpenSslEngine.java:595)
    at io.netty.handler.ssl.ReferenceCountedOpenSslEngine.unwrap(ReferenceCountedOpenSslEngine.java:1202)
    at io.netty.handler.ssl.ReferenceCountedOpenSslEngine.unwrap(ReferenceCountedOpenSslEngine.java:1324)
    at io.netty.handler.ssl.SslHandler$SslEngineType$1.unwrap(SslHandler.java:201)
    at io.netty.handler.ssl.SslHandler.unwrap(SslHandler.java:1380)