Occasional PostgreSQL "Duplicate key value violates unique constraint" error from Go insert

I have a table with the unique constraint

CREATE UNIQUE INDEX "bd_hash_index" ON "public"."bodies" USING btree ("hash");

I also have a Go program that takes "body" values on a channel, filters out the duplicates by hashing, and inserts only the non-duplicates into the database. Like this:

import (
    "crypto/md5"
    "database/sql"
    "encoding/hex"
    "log"
    "strings"
    "time"
)

type Process struct {
    DB                  *sql.DB
    BodiesHash          map[string]bool
    Channel             chan BodyIterface
    Logger              *log.Logger
}

func (pr *Process) Run() {
    bodyInsert, err := pr.DB.Prepare("INSERT INTO bodies (hash, type, source, body, created_timestamp) VALUES ($1, $2, $3, $4, $5)")
    if err != nil {
        pr.Logger.Println(err)
        return
    }
    defer bodyInsert.Close()

    hash := md5.New()

    for p := range pr.Channel {
        nowUnix := time.Now().Unix()

        bodyString := strings.Join([]string{
            p.GetType(),
            p.GetSource(),
            p.GetBodyString(),
        }, ":")
        hash.Write([]byte(bodyString))
        bodyHash := hex.EncodeToString(hash.Sum(nil))
        hash.Reset()

        if _, ok := pr.BodiesHash[bodyHash]; !ok {
            pr.BodiesHash[bodyHash] = true

            _, err = bodyInsert.Exec(
                bodyHash,
                p.GetType(),
                p.GetSource(),
                p.GetBodyString(),
                nowUnix,
            )
            if err != nil {
                pr.Logger.Println(err, bodyString, bodyHash)
            }
        }
    }   
}

But periodically I get the error

"pq: duplicate key value violates unique constraint "bd_hash_index""

in my log file. I can't image how it can be, because I check the hash for uniqueness before I do an insert. I am sure that when I call go processDebugBody.Run() the bodies table is empty.

The channel was created as a buffered channel with:

    processDebugBody.Channel = make(chan BodyIterface, 1000)