Using Dirichlet distribution in Stan

Question

I'm testing the Dirichlet distribution with pyStan 3.7. I'm getting a warning that alpha has two prior, I don't get why.

data {
  int<lower=1> T;  
  int<lower=1> K;  
  int y[T, K];     
}

parameters {
  simplex[K] theta[T];   
  real alpha; 
}

model {
  alpha ~ cauchy(0, 1);  
  
  for (t in 2:T) {
    theta[t] ~ dirichlet(alpha + theta[t-1]);
  }
}